KR100692612B1 - Producing method of spherical silicone fine particles - Google Patents

Abstract

A method of effectively producing micro-sized spherical silicone particles that have high roundness, are very small and have reproducibility is provided. A method of producing spherical silicone fine particles comprises: a step(A) of hydrolyzing a silane compound of a formula 1, R^1Si(X)3, under an acidic condition to obtain a water-soluble or alcohol solution, wherein R^1 is a monovalent group selected from the group consisting of substituted or non-substituted alkyl group, alkenyl group and phenyl group, and X are same or different groups capable of being hydrolyzed ; a step(B) of adding an aqueous alkaline solution to the water-soluble or alcohol solution, and irradiating ultrasonic waves into the solutions to mix the solutions; and a step(C) of performing the polycondensation of the mixed solution in the steady state without agitation to form spherical silicon fine particles.

Description

Producing method of spherical silicone fine particles

The present invention relates to a method for producing spherical silicon fine particles. More specifically, the present invention relates to a method for effectively producing spherical silicon fine particles having an average particle diameter of 2 μm or less.

The method described in Japanese Patent Laid-Open No. 63-77940 is commonly known as a method for producing spherical silicon fine particles. The production process is a mixture of an organotrialkoxysilane and / or a partial hydrolysis product thereof or an organotrialkoxysilane and / or a partial hydrolysis product thereof with an organic solvent and an ammonia and / or amine and organic as a lower layer. Hydrolysis and polycondensation of the organotrialkoxysilane and / or partial hydrolysis products thereof at the interface between the mixed liquid with the solvent yields spherical silicon fine particles in which the individual particles are independently truly spherical in shape. The properties of the silicon fine particles obtained by this method are spherical in form of particles, high hydrophobicity, low condensation ability, and low specific gravity. These microparticles are used as modifying additives to impart lubricating properties, improve dispersibility and provide light scattering action to paints, plastics, rubber, cosmetics and paper. However, such spherical silicon fine particle manufacturing method is complicated to maintain the reaction interface where the organotrialkoxysilane and / or partial hydrolysis products thereof are hydrolyzed and polycondensed. As a result, there is a problem of low productivity with respect to time and capacity of the apparatus.

In order to solve these problems, the aqueous / alcohol solution of organosilanetriol obtained by hydrolyzing the organoalkoxysilane and / or partial condensation products thereof is optionally diluted, an aqueous alkaline solution is added thereto, followed by mixing and the resulting A method for producing spherical silicon fine particles comprising maintaining the mixture in a steady state has been proposed (Japanese Patent Laid-Open No. 2000-186148). According to this method, hydrolysis and polycondensation are carried out in a homogeneous reaction state to obtain spherical silicon fine particles. Thus, the complicated maintenance of the interface at which hydrolysis and polycondensation is performed, as in the conventional art, is not necessary, and the productivity for time and device capacity can be greatly improved. According to the method, the average particle diameter of the spherical silicon fine particles obtained can be adjusted within the range of 1 to 10 mu m, and the properties of the spherical silicon fine particles obtained are narrower in size distribution than the conventional techniques. Spherical silicon fine particles are used in combination with plastics for light diffuser panels of liquid crystal displays and to impart lubricating properties to improve the running stability of high quality video tapes. However, this method has a problem that it is difficult to obtain fine particles having an average particle size of less than 1 mu m. That is, according to the method, it is necessary to reduce the concentration of the reaction product or a dilution step in order to obtain particles having a relatively small average particle size. Thus, in the production of particles with relatively small average particle sizes, the productivity with respect to time and capacity of the device is lower than when obtaining particles with relatively large average particle sizes.

In addition, according to US Pat. No. 4,449,49, a small amount of tetraalkoxysilane was added to the organotrialkoxysilane to obtain spherical silicon fine particles having high productivity. In this method, it was claimed that silicon fine particles of 0.1-5㎛ size could be obtained by selecting organic groups and water concentrations for the Si concentration in the reactants. The change is very big disadvantage.

There is also known a method for producing spherical silicon fine particles by adding an organic solvent, such as isopropyl alcohol, t-butanol, 1-hexanol, and the like, which is well soluble in water after hydrolysis of the organotrialkoxysilane before polycondensation (JP 2003-2973, JP 2003-183395, JP 2003-183396). However, even in this case, there is a problem that the size is very large or the size deviation is severe.

SUMMARY OF THE INVENTION The present invention has been made to solve this problem, and an object of the present invention is to provide a method for effectively producing spherical silicon fine particles having high roundness, small size, and reproducibility.

As a result of intensive studies to achieve the above object, the above objects are achieved by the aqueous solution of silanol and / or partial condensation products thereof obtained by hydrolyzing organotetraalkoxysilane in which 3-4 hydrolyzable groups are bonded to Si. It can be achieved by adding an alkaline aqueous solution to the alcohol solution, mixing the resulting mixture quickly and homogeneously, but irradiating with ultrasound to speed up the mixing and polycondensation, and then keeping the obtained mixture static without stirring Revealed.

The present invention has surprisingly completed the present invention by discovering that the addition of a means for irradiating and stirring ultrasonic waves at the initial stage of polycondensation enables the production of microparticles of very small, reproducible, and improved roundness. The initial meaning of the polycondensation means that it is within 30 minutes from the start of the polycondensation. When the time of applying the ultrasonic wave is more than 30 minutes, the effect of homogeneous mixing by the ultrasonic wave is inferior and rather agglomeration of particles occurs. That is, the ultrasonic waves should be applied within the initial 30 minutes of the polycondensation reaction to prepare fine particles having a uniform particle size.

The spherical silicon fine particles of the present invention are obtained by the polycondensation reaction of the silane compound of formula (1). In addition, the silane compound of Formula 2 may be further added to the silane compound of Formula 1, or an alcohol having 3 or more carbon atoms or a mixture thereof may be further added.

Formula 1 R ¹ Si (X) ₃

Si (X) ₄

In Formula 1 above, R ¹ is a monovalent group selected from the group consisting of substituted or unsubstituted alkyl groups, alkenyl groups and phenyl groups, and X is the same or different hydrolyzable group.

Method for producing spherical silicon fine particles according to the present invention

(A) hydrolyzing the silane compound of Formula 1 under acidic conditions to obtain an aqueous or alcoholic solution of silanol and / or partial hydrolysis products thereof;

(B) adding an alkaline aqueous solution to the aqueous or alcoholic solution of the product, followed by mixing with ultrasonic waves; And

(C) polycondensing the mixture under a steady state without stirring to form spherical silicon fine particles.

In another embodiment of the present invention, in the step (A), the silane compound of Formula 2 is further added so that the ratio of R ¹ / Si is 0.5 to 1.0 or less, or an amount of 1 to 80 weights per 100 weights of silanes containing 3 or more carbon atoms It may be further added to, and both the tetraalkoxysilane and three or more carbon atoms may be added.

Examples of X in Formula 1 or 2 may be an alkoxy group of Formula OR ² and Formula 1 or 2 may be an organosiloxane partially containing halogen. In the present invention, the silane compound of Formula 1 is preferably an organotrialkoxysilane of Formula 3, and the silane compound of Formula 2 is preferably a tetraalkoxysilane of Formula 4.

Formula ¹ R ¹ Si (OR ² ) ₃

Si (OR ² ) ₄

In Formulas 3 and 4, R ¹ is a monovalent group selected from the group consisting of a substituted or unsubstituted alkyl group, an alkenyl group, and a phenyl group, and R ² is the same or different substituted or unsubstituted alkyl group.

Hereinafter, the present invention will be described in detail.

In the silane compound of Formula 1 or Formula 3 used in the present invention, R ¹ is a monovalent group selected from the group consisting of substituted or unsubstituted alkyl groups, alkenyl groups, and phenyl groups, and examples of R ¹ include 1 to 6 carbon atoms. Alkyl groups, alkenyl groups having 2 to 6 carbon atoms, and phenyl groups. These may be used alone or as a mixture. Methyl groups are preferred because of their good reactivity. X in Formula 1 or 2 is the same or different hydrolyzable group, an example of X being an alkoxy group of Formula OR ² , which may be an organosiloxane that partially contains a halogen such as chlorine. Examples of R ² of the alkoxy group of OR ² in Formulas 3 and 4 include alkyl groups such as methyl, ethyl, propyl or butyl and alkoxy substituted hydrocarbon groups such as 2-methoxyethyl or 2-ethoxyethyl do. Methyl, ethyl and 2-methoxyethyl are preferred due to the high rate of hydrolysis. Examples of such methyltrialkoxysilanes include methyltrimethoxysilane, methyltriethoxysilane, methyltri-n-propoxysilane, methyltriisopropoxysilane and methyltris (2-methoxyethoxy) silane. . These may be used alone or as a mixture. In addition, examples of the tetraalkoxysilane of the formula (4) include tetramethoxysilane, tetraethoxysilane, tetra-n-propoxysilane, tetraisopropoxysilane and tetra (2-methoxyethoxy) silane, These compounds may be used alone or as a mixture. In reaction of the manufacturing method of the spherical silicon microparticles of this invention, when organosilane which halogen is couple | bonded with Si is used, hydrolysis rate is too fast and it is difficult to control reaction. Therefore, it is preferable to use the organotrialkoxysilane of the formula (3) and the tetraalkoxysilane of the formula (4).

In the first step (A) of the preparation method according to the present invention, when the tetraalkoxysilane of the formula (4) is further prepared, the organotrialkoxysilane of the formula (3) and the tetraalkoxysilane of the formula (4) are added under acidic conditions Decomposition yields an aqueous or alcoholic solution of the organosilanol and / or its partial hydrolysis products. The mixing ratio of the organotrialkoxysilane of the formula (3) to the tetraalkoxysilane of the formula (4) is 0.5 to 1.0 or less, preferably 0.8 to 1.0, more preferably 0.9 to 1.0 in terms of the ratio of R ¹ / Si. When R ¹ / Si ratio of the reduction, the smaller the particle size, if the nature of the organic material reduces excessively R ¹ / Si ratio is reduced is obtained, only the gel-like material instead of the particles obtained. Therefore, in order to achieve the object of the present invention, the mixing ratio should be adjusted according to the purpose of use.

In the first step (A) of the preparation method according to the present invention, an alcohol having 3 or more carbon atoms may be added to the organotrialkoxysilane, and the particle size may be reduced by adding the alcohol. Alcohols added are primary alkyl alcohols such as 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, 1-heptanol, 1-octanol, 1-decanol, 2-propanol, 2-butanol , Secondary alkyl alcohols such as 2-pentanol, tertiary alcohols such as 1,1-dimethyl ethanol, cyclic alcohols such as cyclopentanol and cyclohexanol, cyclocyclic alcohols such as cyclohexyl methanol, and 2-phenyl ethanol Aryl substituted alcohols such as these may be used. 1-propanol, 1-butanol, 1-pentanol, 1-hexanol and the like are suitable, 1-80 weight alcohol per 100 weight organotrialkoxy silane, and 5-25 weight alcohol are more suitable. Too little alcohol makes it difficult to expect the effects of alcohol, and too much alcohol results in low productivity due to too thin reactants. In addition, methanol and ethanol have similar physical properties to water, and have a weak effect as an additional solvent.

The present invention can be applied as an acid in which an organic acid or an inorganic acid is used in the hydrolysis reaction. Examples of organic acids include formic acid, acetic acid, propionic acid, oxalic acid and citric acid. Acetic acid is particularly preferred in view of ease of control of the partial condensation reaction of the resulting organosilanol. Any inorganic acid may be used as the inorganic acid for hydrolysis, so long as no impurities such as ionic substances which limit the use of the finally obtained silicon fine particles remain. Hydrochloric acid is particularly preferred because it is easy to obtain. Organochlorosilanes such as organotrichlorosilanes or hydrogen chloride contained in trace amounts as impurities of silanes can be used as catalysts or catalyst precursors. The amount of acid used depends on the type of silane and acid and the amount of water used. However, the acid concentration of the aqueous acid solution prepared by dissolving the acid in water is preferably 1 × 10 ⁻⁵ to 100 × 10 ⁻⁵ N. If the acid concentration is less than 1 × 10 ⁻⁵ N, hydrolysis does not proceed properly and the rate is too slow. On the other hand, when it exceeds 100x10 <^-5> N, it becomes difficult to control a hydrolysis reaction, and it is difficult to arbitrarily adjust the average particle size of the finally obtained silicon microparticles | fine-particles. The amount of water used for hydrolysis is preferably 20 to 150 mol per mol of silane. When the amount of water is less than 20 mol, agglomeration tends to occur between the formed fine particles. On the other hand, when it exceeds 150 mol, the yield of the silicon fine particles finally obtained decreases and its productivity also decreases. The water used for hydrolysis should preferably be free of impurities such as ionic materials which are high in purity and limit the use of the final microparticles obtained. That is, water having an electrical conductivity of 50 mS / m or less is preferably used. In particular, deionized water having an electrical conductivity of 2 mS / m or less is more preferably used. The hydrolysis reaction is not particularly limited. However, in order to effectively obtain organosilanol and / or partial condensation products thereof, the reaction is preferably carried out with a temperature in the range of −10 to 60 ° C. maintained for 1 to 6 hours. Too low a temperature causes the reaction to be too slow and a high temperature increases the particle size. Thus, according to step (A), the silane used is hydrolyzed such that the organosilanol and / or its partial condensation product is reacted with an alcohol or substituted formed by reaction with an additional amount of water other than the water consumed during hydrolysis. Obtained in the form of a solution formed by dissolving it in a mixture comprising an alcohol.

The second step (B) of the preparation method of the present invention is an aqueous or alcoholic solution of the organosilanol and / or partial hydrolysis products thereof obtained in the first step (A) (hereinafter referred to as "silanol solution" After the addition of an alkaline aqueous solution, the step of irradiating with ultrasonic waves and mixing, and the third step (C) of the manufacturing method of the present invention is the polycondensation by leaving the mixture homogeneously mixed in the second step in a steady state without stirring A step of obtaining spherical silicon fine particles by the sum reaction.

The alkaline aqueous solution exhibits basic properties and acts as a neutralizer for the acid used in the first step and also as a catalyst for the polycondensation reaction. Examples of alkaline materials of such alkaline aqueous solutions include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, ammonia and organic amines such as monomethylamine and dimethylamine. Of these compounds, even traces of impurities which limit the use of spherical silicon fine particles from which ammonia and organic amines are produced remain, which is preferable. Also particularly preferred is ammonia, which is easily removable. The alkaline aqueous solution is used in an amount that neutralizes the acid and also effectively acts as a catalyst for the polycondensation reaction. In addition, the reaction mixture prepared by rapid addition and homogeneous mixing is used in an amount capable of maintaining a time that can be allowed to settle quickly in a steady state before the formation and attachment of the silicon fine particles. That is, when the amount of alkaline aqueous solution used exceeds the amount necessary for neutralization of the acid, an aqueous ammonia solution having a concentration of 1 to 5% is used in an amount of 0.05 to 3 weights per 100 weights of the silanol solution obtained in the first step. do. In a second step (B), the silanol solution is charged to the reaction vessel and the alkaline solution is added to the silanol solution. The resulting solution is mixed quickly and homogeneously by any method such as stirring, but irradiated with ultrasonic waves. The addition method is not particularly limited as long as the addition can be carried out effectively within the minimum mixing time. The alkaline solution can be added from above the silanol solution or introduced into the silanol solution through a nozzle. The time for mixing and irradiating the ultrasonic wave is a time required for dissolving or homogeneously mixing the alkali catalyst in the reaction mixture, suitably a short time within 30 minutes, more preferably within 10 minutes. For example, the mixing time including the addition time is 0.1 to 10 minutes, more preferably 0.5 to 3 minutes at a temperature of -10 to 30 ° C, preferably -10 to 20 ° C. If the ultrasonic irradiation time is too long, entanglement of fine particles may occur, and if it is too short, it is difficult to expect the ultrasonic irradiation effect. Ultrasonic irradiation can raise the temperature of the reaction mixture and the temperature rise is not preferable because of the very rapid polycondensation, resulting in entanglement between the particles and wider particle size distribution. On the other hand, if the temperature is too low, it is not easy to handle the reaction system containing an aqueous solution or alcohol, and the mixing time is very long and inefficient. The temperature can be controlled by any method, and the temperature of the reactants can be kept constant by flowing coolant using a cooling coil or kept at a constant temperature by using a cooling bath. Since the temperature may rise rapidly by ultrasonic waves, it is efficient to prevent the temperature of the reactants from rising sharply by inducing rapid contact between the reactants and the refrigerant by flowing the cooling water using the cooling coil. Ultrasonic irradiation may be continued during the mixing period, or periodically, or only at the beginning of mixing.

Ultrasound is a sound wave having a frequency of more than 16000 Hz, irrespective of any sound wave of any frequency, but is suitable for industrial use, 20000 and 40000 Hz. Ultrasonic baths, such as ultrasonic cleaners, can be used, but ultrasonic waves are dispersed and inefficient, so ultrasonic processors with ultrasonic irradiation tips called horns can irradiate the reactants with ultrasonic waves. More suitable.

The third step of the present invention is a step of completing the polycondensation by irradiating the ultrasonic wave in the second step and allowing the homogeneously mixed mixture to stand without stirring. The process of standing the mixture can provide spherical silicon particles of the desired average particle size and can also provide good efficiency over time and capacity of the device. For example, this process is carried out for 2 to 24 hours, preferably 2 to 10 hours while maintaining the mixing temperature of the second step. By carrying out the polycondensation in the third step (C), spherical silicon fine particles can be obtained in the water / alcohol mixture in the form of a dispersion or a sol. Spherical silicon particles in the form of a dispersion or sol obtained according to the invention can be used as is. In addition, if necessary, spherical silicon particles can be recovered as finely divided powder by suitable treatment such as filtration, drying and grinding. The average particle diameter of the silicon fine particles is determined by the ratio of solvents such as tetraalkoxysilane and / or alcohol used for hydrolysis, the amount of water used for hydrolysis, the amount of alkali used for polycondensation, and ultrasonic irradiation at the initial stage of polycondensation. It can be adjusted by varying the time and degree.

The invention is illustrated in detail by the following examples and comparative examples, but the scope of the invention is not limited by the examples. In the examples below, the concentration percentages are based on weight. Evaluation of the microparticles shown in the examples was made from scanning electron micrographs, and the roundness (ratio of the short diameter of the particles / the long diameter of the particles) and the average diameter of the particles (µm) were measured from 50 samples. Is due to the average.

Example 1

First step

19.08 g of purified deionized water, passed through the ion exchange resin, is charged to a reaction vessel equipped with a thermometer, a cooling coil and a stirrer. 0.010 g of 0.35% hydrochloric acid is added thereto to give an acid concentration of 5 × 10 ⁻⁵ N. The resulting acidic solution was added with 2.641 g of methyltrimethoxysilane and 0.154 g of tetramethoxysilane so that the molar ratio of methyl group / Si was 0.95 while stirring at 10 ° C. As a result, hydrolysis proceeds and stirring continues for an additional 5 hours and the resulting reaction mixture is kept at 10 ° C. to obtain a silanol solution.

2nd step

While stirring the silanol solution obtained in the first step at 10 ° C., 0.085 g of a 2.8% aqueous ammonia solution was added thereto, and an ultrasonic processor (Sonics, model VC-750, maximum output 750W, titanium tip) generated 80% output. Start to stir for 3 minutes and irradiate the reactants with ultrasonic waves to ensure uniform mixing.

3rd step

Agitation is stopped and the resulting reaction mixture is left for 4 hours. The resulting solution is centrifuged to yield a wet cake. The wet cake thus obtained was dried at 150 ° C. for 12 hours to give a white powder. Observation of the obtained white powder with an electron microscope showed that the particles each had an independent and true spherical form. The average particle size of the white powder is 1.2 μm and the roundness is 0.98. Detailed reaction conditions and results are summarized in Table 1.

Example 2

Following the same procedure as in Example 1, only the amounts of methyltrimethoxysilane and tetramethoxysilane were changed. That is, the molar ratio of methyl group / Si was set to 0.90. The obtained white powder had an average particle size of 0.9 μm and a roundness of 0.98. Detailed reaction conditions and results are summarized in Table 1.

Example 3

Following the same procedure as in Example 1, only the amounts of methyltrimethoxysilane and tetramethoxysilane were changed. That is, the molar ratio of methyl group / Si was set to 0.97. The obtained white powder had an average particle size of 1.3 mu m and a roundness of 0.98. Detailed reaction conditions and results are summarized in Table 1.

Example 4

Following the same procedure as in Example 1, only methyltrimethoxysilane was used instead of methyltrimethoxysilane and tetramethoxysilane. That is, the molar ratio of methyl group / Si is 1.0, and the average particle size of the obtained white powder is 1.5 mu m and the roundness is 0.98. Detailed reaction conditions and results are summarized in Table 1.

Example 5

Following the same procedure as in Example 4, 0.313 g of 1-hexanol was further used in addition to 2.78 g of methyltrimethoxysilane. That is, the weight ratio of alcohol / organotrialkoxysilane was 0.113, and the average particle size of the obtained white powder was 1.4 mu m and roundness was 0.98. Detailed reaction conditions and results are summarized in Table 1.

Comparative Example 1

Following the same procedure as in Example 1, an aqueous ammonia solution was added to the hydrolyzed silanol solution without ultrasonic irradiation. The average particle size of the white powder obtained was 2.2 μm, roundness was 0.97, and the surface was very rough. Detailed reaction conditions and results are summarized in Table 1.

Comparative Example 2

Aqueous ammonia solution was added to the hydrolyzed silanol solution following the same procedure as in Example 4 but without ultrasonic irradiation. The average particle size of the white powder obtained is 3.2 μm on average and roundness is 0.97. Detailed reaction conditions and results are summarized in Table 1.

[Table 1] Spherical Silicon Manufacturing Conditions and Obtained Silicon Properties

According to the present invention, it is possible to effectively produce spherical silicon particles of very round shape having a very small particle size and a roundness of 0.98 or more. The method of the present invention is most effective for applying spherical silicon particles having an average particle diameter of 1.5 mu m or less which cannot be effectively produced according to conventional methods. The spherical silicon particles obtained according to the production method of the present invention are insoluble in the inorganic solvent and do not melt. In addition, its surface is excellent in water repellency and lubricating properties. In addition, the spherical silicon particles obtained have a specific gravity smaller than that of the inorganic powder, while being excellent in heat resistance, low cohesiveness and excellent dispersibility in comparison with the organic powder. Therefore, the spherical silicon particles obtained due to these properties are useful as fillers or lubrication improvers in paints, plastics, rubber, paper and cosmetics, and also as additives for plastic modification used to impart light scattering action to optical liquid crystal displays. .

Claims (5)

(A) hydrolyzing the silane compound of Formula 1 under acidic conditions to obtain an aqueous or alcoholic solution; (B) adding an alkaline aqueous solution to the aqueous or alcoholic solution of the product and irradiating with ultrasonic waves to mix; And (C) polycondensing the mixture under steady state without stirring to form spherical silicon fine particles; Method for producing spherical silicon fine particles comprising a. Formula 1 R ¹ Si (X) ₃ (Wherein R ¹ is a monovalent group selected from the group consisting of substituted or unsubstituted alkyl groups, alkenyl groups and phenyl groups, and X is the same or different hydrolyzable group.) The method of claim 1, In the step (A), in addition to the organotrialkoxysilane of the general formula (1), a method for producing spherical silicon microparticles, characterized in that the addition of a silane compound of the formula (2), an alcohol having 3 or more carbon atoms or a mixture thereof. [Formula 2] SiX ₄ Wherein X is the same or different hydrolyzable group. The method of claim 2, The molar ratio of R ¹ / Si is 0.5 to 1.0 method for producing spherical silicon fine particles. The method of claim 3, The molar ratio of R ¹ / Si is 0.9 to 1.0 method for producing spherical silicon fine particles, characterized in that. (A) a silane compound consisting of a tetraalkoxysilane of the formula (4) such that the ratio of the organotrialkoxysilane of the formula (3) and R ¹ / Si is 0.9 to 1.0; Hydrolyzing the composition comprising at least one alcohol component selected from and 20 to 150 mol of water per mol of the silane compound under acidic conditions to obtain an aqueous or alcoholic solution of the hydrolysis product; (B) adding an alkaline aqueous solution to the aqueous or alcohol solution and irradiating and mixing ultrasonic waves; And (C) polycondensing the mixture under steady state without stirring to form spherical silicon fine particles; Method for producing spherical silicon fine particles comprising a. Formula ¹ R ¹ Si (OR ² ) ₃ Si (OR ² ) ₄ (Wherein R ¹ is a monovalent group selected from the group consisting of substituted or unsubstituted alkyl groups, alkenyl groups and phenyl groups, and R ² is the same or different substituted or unsubstituted alkyl group.)