WO2008125787A1

WO2008125787A1 - Method for preparing a catalyst and use thereof in a continuous method for the transesterification of (meth)acrylic esters

Info

Publication number: WO2008125787A1
Application number: PCT/FR2008/050392
Authority: WO
Inventors: Alain Riondel; Gaëlle BAQUEY; Hervé DELEUZE; Marc Birot
Original assignee: Arkema France
Priority date: 2007-03-16
Filing date: 2008-03-07
Publication date: 2008-10-23
Also published as: FR2913612A1; FR2913612B1

Abstract

The invention relates to the synthesis of (meth)acrylic monomers and more particularly relates to a method for preparing a catalyst of the alkyl titanate type born on a polymeric material, and to the use thereof as a transesterification heterogenous catalyst for the continuous synthesis of (meth)acrylic esters.

Description

PROCESS FOR THE PREPARATION OF A CATALYST AND USE THEREOF IN A CONTINUOUS PROCESS OF TRANSESTERIFICATION OF ESTERS

(METH) ACRYLIC

The present invention relates to the synthesis of (meth) acrylic monomers, and more particularly to a process for the preparation of an alkyl titanate catalyst supported on a polymeric material, and its use as a heterogeneous transesterification catalyst for synthesis. continuous (meth) acrylic esters.

A commonly used method for producing (meth) acrylic esters is transesterification. It is known, for example from GB 960,005, EP 298,867, EP 619,309 or EP 960,877, to prepare the (meth) acrylic esters of formula (I):

in which R is a hydrogen atom or a methyl group, and R 1 can be a linear or branched alkyl radical, or a cyclic aliphatic radical, aryl, alkyl-aryl or aryl-alkyl, which can contain heteroatoms, according to a method of transesterification by reaction of an alkyl (meth) acrylate of formula (II):

in which R has the abovementioned meaning and R ₂ may be a linear or branched alkyl group having from 1 to 4 carbon atoms, with an alcohol of formula

RrOH in which R 1 has the meaning mentioned above. During the synthesis is generated light alcohol R ₂ -OH, which is removed as an azeotrope with the light alkyl (meth) acrylate (II). The synthesis of (meth) acrylic esters by transesterification is generally carried out in the presence of a catalyst which can be homogeneous or heterogeneous. The choice of the catalyst depends on various criteria, in particular on the nature of the (meth) acrylate of alkyl (II) or of the alcohol (III) used, but also on the nature of the process, such as batch process or continuous process. In addition to the criteria of efficiency and selectivity, other factors may be involved, such as commercial availability, price or toxicity of the catalyst.

As examples of homogeneous catalysts described in the literature for catalyzing the preparation of (meth) acrylic esters by transesterification, mention may be made in particular of:

Acidic catalysts, such as methanesulphonic acid, or para toluenesulphonic acid: these catalysts have the drawback of being not very selective and corrosive. - Basic catalysts, such as alkaline or alkaline earth salts: these catalysts are active but are not very selective. Titanium, zirconium, iron, zinc or calcium chelates with 1,3-dicarbonyl compounds such as Ti, Zr, Fe, Zn or Ca acetylacetonates: these compounds are active and selective, but they produce acetylacetone and are sensitive to the presence of diols 1, 2 or 1, 3.

The tin derivatives, such as tin dialkyloxides, tin dialkyldialcoxides and tin dialkyldiesters, in particular tin t-butyloxide (DBTO and its counterparts), are generally active and selective. However, with these catalysts there is the problem of their elimination in the distillation residues because of the toxicity of tin.

Titanium alkoxides, such as tetraalkyl (ethyl, n-propyl, isopropyl, n-butyl, etc.) titanates, or else dimethylaminoethyl titanate or titanium phenolate: these are generally active and selective catalysts, but sensitive to water.

Homogeneous catalysis has the disadvantage of having to separate the catalyst from the reaction medium at the end of the reaction; in addition, it sets the implementation of perfectly stirred reactor-type installations involving rotating machines which consume a great deal of electricity and maintenance costs, and which may also give rise to problems of safety and reliability. Heterogeneous catalysts are also used to catalyze the transesterification reaction. EP 557 131 discloses a process for preparing a polymer supported catalyst which comprises copolymerizing a slurry mixture comprising a vinyl monomer, a crosslinking monomer and a polymerizable chelated metal species. The catalyst obtained is suitable for catalyzing saturated or unsaturated ester exchange reactions, such as the transesterification of (meth) acrylic esters.

In the article Journal of Polymer Science, Part A 38 (2000) pp 2879-2886, Deleuze et al describe the synthesis of porous supports based on N- (p-hydroxyphenyl) maleimide or N- (3-4-dihydroxybenzyl) ) maleimide, on which are grafted titanates such as isopropyl titanate or titanium chloride. The catalysts thus obtained were tested for the transesterification of methyl methacrylate with 2-ethylhexanol according to a batch process. After several recycles, the catalysts show a loss of activity by degaking the titanate in the reaction medium. Deleuze et al in Polymer 39 (1998) pp 6109-6114 describe the preparation of titanates supported on polymers and their use as transesterification catalysts. Macro-porous beads of poly (4-acetoxystyrene-co-styrene-co-divinylbenzene) are firstly prepared by suspension polymerization of the various monomers in 2-ethylhexanol at 80 ^° C. for 8 hours. After washing with water, ethanol and acetone, then drying under vacuum, the polymer particles are then hydrolysed in dioxane in the presence of hydrazine hydrate at 80 ^° C. The hydrolysis lasts 2 days. After separation, washing and drying, the poly (4-hydroxystyrene-co-styrene-co-divinylbenzene) beads obtained are grafted with a chlorinated alkyl titanate; the reaction is carried out in chloroform in the presence of triethylamine. The catalyst thus obtained leads to yields of 2-ethylhexanol methacrylate of the order of 50% after 24 hours. during the transesterification of methyl methacrylate with 2-ethylhexanol. The preparation of this catalyst has the disadvantage of requiring many steps that are long and tedious.

The applicant company has now discovered that it is possible to more easily prepare catalysts based on titanates supported on polymers, by directly grafting an alkyl titanate on a polymer functionalized with ester functions and preferably with acetoxyl functions. . The grafting consists of a direct exchange between the acetoxyl function and the alkyl titanate and does not require an intermediate step of hydrolysis of the acetoxyl functions.

The subject of the present invention is therefore a process for the preparation of an alkyl titanate catalyst supported on a polymeric material comprising the following steps:

(a) preparation of support beads by suspension polymerization of at least two monomers having ethylenic unsaturations, at least one of the monomers having at least one acetoxyl function,

(b) grafting an alkyl titanate from the acetoxyl function of the polymer obtained in step (a).

The subject of the invention is also a process for the continuous synthesis of (meth) acrylic esters of formula (I):

in which R is a hydrogen atom or a methyl group, and R 1 is a linear or branched alkyl radical, or a cyclic aliphatic radical, aryl, alkyl-aryl or aryl-alkyl, containing from 4 to 40 carbon atoms, or a linear or branched alkyl radical containing at least one heteroatom and from 3 to 40 carbon atoms, by reaction of an alkyl (meth) acrylate of formula (II):

in which R has the abovementioned meaning and R 2 is a linear or branched alkyl group having from 1 to 3 carbon atoms, with an alcohol of formula (III): R 1 -OH in which R 1 has the abovementioned meaning, in the presence of a alkyl titanate catalyst supported on a polymeric material prepared according to the method described above.

The use of a heterogeneous catalyst of alkyl-titanate type supported on a polymeric material in a continuous process of transesterification for the synthesis of (meth) acrylic esters has many advantages over the implementation in a process. in batch. Under the conditions of a batch process, the heterogeneous catalyst is subjected to mechanical agitation and consequently undergoes a loss of activity by attrition, which is even more important when it comes to graft polymer. In addition, the removal of the catalyst at the end of synthesis requires an additional filtration step that is costly in time and investment. In a continuous process, the heterogeneous catalyst is generally used in a fixed bed, which greatly reduces disruption problems related to de-grafting. This results in a gain in implementation costs by improving the catalyst recycling rate and reducing the burning of catalytic residues. Selectivity and productivity are improved with reduced maintenance costs. Compared with homogeneous catalysis, the installation is simplified because of the removal of a distillation column necessary for separating the catalyst from the crude reaction product. Moreover, in the case of monomers with a very high boiling point (> 250 ° C. under atmospheric pressure), homogeneous catalysis leads to the manufacture of a monomer containing the catalyst, which in some cases may be detrimental to the value of the catalyst. application of the monomer.

Other features and advantages of the invention will become more apparent on reading the detailed description which follows. In the process for preparing the catalyst according to the invention, the first step (a) consists in preparing a polymer by suspension polymerization of at least two ethylenically unsaturated monomers, at least one of the monomers having at least one acetoxyl function. . As monomers having at least one acetoxyl function, there may be mentioned 4-acetoxystyrene, N- (p-acetoxyphenyl) maleimide, 3-methoxy-4-acetoxystyrene, 3,5-dimethoxy-4-acetoxystyrene, 3, 5-diacetoxystyrene, 3,4-diacetoxystyrene.

The monomer having acetoxy functional (s) is present in an amount of from 10 to 30 mol% in the polymer.

Mention may be made, as monomers having ethylenic unsaturations, of styrene, divinylbenzene, trivinylbenzene, divinylethylbenzene, divinylxylene, divinylnaphthalene, divinylchlorobenzene, divinylpyridine, trimethylolpropane trimethacrylate, ethylene glycol diacrylate, divinyl ether diethylene glycol, bisphenol dimethacrylate, pentaerythritol tetra and trimethacrylate, diallyl phthalate, triallyl pentaerythritol, divinyl sulfone, divinyl ketone, divinyl sulfide, diallyl carbonate, 1,2,4- trivinylcyclohexane, 1,5-hexadiene, 1,5,9-decatriene, 1,9-decadiene, 1,5-heptadiene and similar compounds. According to one embodiment of the invention, a terpolymer is prepared, preferably based on styrene, divinylbenzene and 4-acetoxystyrene. The preferred terpolymer is styrene / divinylbenzene / 4-acetoxystyrene in molar proportions of between 20% to 70% for styrene, 20% to 50% for divinylbenzene and 10% to 30% for 4-acetoxystyrene and preferably % for styrene, 50% for divinylbenzene and 20% for 4-acetoxystyrene.

The suspension polymerization is carried out according to the conditions well known to those skilled in the art.

The organic phase, consisting of the mixture of the monomers may be diluted in an organic solvent such as for example 2-ethylhexanol, toluene, cyclohexane or a mixture of these solvents. The copolymerization is of radical type initiated by organic peroxides or azo compounds. Useful radical initiators are lauroyl peroxide, benzoyl peroxide, azobis isobutyronitrile, azobis (2,4-dimethylvaleronitrile), and similar compounds. The preferred initiator is azobis isobutyronitrile (AIBN).

The organic phase is added in an aqueous phase which may contain conventional dispersing agents such as xanthan gum, poly (diallyl dimethyl ammonium chloride), polyacrylic acid (or its salts), polyacrylamide, polyacrylamide, polyvinyl alcohol, polyvinylpyrrolidone, acacia gum, or other types of stabilizing agents well known to those skilled in the art. The preferred suspending agent is acacia gum.

The polymerization temperature is generally between 50 and 90 ^° C., preferably between 65 ° C. and 85 ° C. The polymerization is carried out with stirring for a period of from 5 to 24 hours.

After cooling, the polymer obtained is subjected to various treatments, such as washing with water, washing with a solvent such as THF, with ethanol, continuous extraction in a Soxhlet type apparatus with water or with a solvent organic. After drying under vacuum, polymer beads are obtained whose size can vary between 0.1 and 1 mm. The size dispersion of the beads depends on various parameters of the suspension such as: the dimensions of the reactor and the stirring blade, the nature of the stabilizing agent, the amount of stabilizing agent, the stirring speed; the influence of these parameters on the size of the beads is well known to those skilled in the art.

The second step (b) of the process according to the invention consists in carrying out an exchange reaction between the acetoxyl functions of the polymer beads (P) and an alkyl titanate to obtain a polymer grafted with an alkyl titanate according to the following reaction: (P) -OC (O) -CH ₃ + Ti (OR ¹ J ₄ - "PO-Ti (OR ¹ J ₃ + CH ₃ -C (O) -OR '

As titanates Ti (OR ') ₄ , it is possible to use, by way of example, tetraalkyl titanates such as tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, tetrabutyl titanate, tetraisobutyl titanate, tetra-2-ethylhexyl titanate, or dimethylaminoethyl titanate or titanium phenolate, and mixtures thereof. Preferably, tetrabutyl titanate or tetraisopropyl titanate is used. The alkyl titanate is preferably dissolved in an organic solvent such as cyclohexane, p-xylene, toluene in proportions ranging from 20/80 to 80/20.

The polymer beads obtained at the end of step (a) are brought into contact with the solution of alkyl titanate at a temperature of between 50 and 150 ^° C. for a period of at least 10 hours.

The molar ratio of alkyl titanate by acetoxyl functions is preferably between 0.5 and 4, preferably between 1 and 3, more particularly between 1, 2 and 2.

One embodiment of the process of the invention consists in introducing the polymer balls in a thermostatically controlled column and in continuously introducing the titanate solution until the column is completely filled, and then left in contact for a period of time. at least 48 hours.

The grafted polymer beads are then washed with the organic solvent to remove excess ungrafted alkyl titanate and remove the alkyl acetate formed.

One method of treatment of the process of the invention is to continuously introduce into the thermostated column containing the grafted beads to be washed, a solvent such as methylene chloride or toluene, then to follow, visually or by spectroscopy, the disappearance staining of the eluate, a control of the excess of ungrafted titanate.

The target substitution rate, expressed as a percentage of acetoxyl functions exchanged, is generally between 80 and 100%.

The graft polymer beads obtained are generally characterized by a mean specific surface area (BET) ranging from 100 to 250 m ² / g and a pore size of 100 to 200 nm. The catalyst obtained according to the process of the invention is advantageously used for the continuous transesterification of light (meth) acrylates of formula (II):

in which R is a hydrogen atom or a methyl group, and R ₂ is a linear or branched alkyl group having from 1 to 3 carbon atoms, with an alcohol of formula (III):

RrOH (III) in which R1 is a linear or branched alkyl radical, or a cyclic aliphatic, aryl, alkyl-aryl or aryl-alkyl radical, containing from 4 to 40 carbon atoms, or a linear or branched alkyl radical containing at least a heteroatom and from 3 to 40 carbon atoms.

As light (II) alkyl (meth) acrylate, it is possible to use methyl (meth) acrylate, ethyl (meth) acrylate, propyl (meth) acrylate. More particularly, methyl methacrylate, methyl acrylate or ethyl acrylate are used.

As examples of alcohols of formula (III), there may be mentioned butanol, 2-ethylhexanol, dimethylaminoethanol, dimethylaminopropanol, diethylaminoethanol, hydroxyethyl imidazolidone, a fatty alcohol having on average 40 carbon atoms; preferably, butanol, 2-ethylhexanol or dimethylaminoethanol are used.

Advantageously, butyl methacrylate and dimethylaminoethanol acrylate are prepared according to the invention.

The catalyst is used in a proportion of 0.005 to 0.5 mole per mole of alcohol (III), preferably 0.005 to 0.05 mole, more particularly in a proportion of 0.007 mole per mole of alcohol (III).

A molar ratio (meth) acrylate of alkyl (II) on alcohol (III) of between 2 and 5, preferably between 2 and 4, is preferably chosen. The reaction temperature is generally between 80 and 130 ^° C .; an overpressure of 2 to 6 bars relative to the atmospheric pressure is generally maintained.

Preferably, the reaction temperature is between 115 and 120 ^° C. and the excess pressure is of the order of 2 bars relative to the atmospheric pressure.

As the polymerization inhibitor added to the reaction medium, phenothiazine, hydroquinone, methoxyphenol, hydroquinone monomethyl ether, diterbutyl para-cresol (BHT), paraphenylene diamine, TEMPO (2, 2,6,6-tetramethyl-1-piperidinyloxy), di-tert-butylcatechol, or TEMPO derivatives, alone or as a mixture, at 100 to 5000 ppm relative to the initial charge, preferably between 500 and 3000 ppm .

The transesterification reaction may be carried out in any apparatus suitable for carrying out a continuous process such as that described in the examples or by any other similar system well known to those skilled in the art.

Surprisingly, it has been observed a good activity of alkyl titanate catalysts supported on polymer beads, resulting in a conversion rate to (meth) acrylic esters generally greater than 60% and stability for at least 15 days.

The following examples illustrate the present invention without, however, limiting its scope.

Examples

EXAMPLE 1 Preparation of a Catalyst According to the Invention Preparation of Support Balls by Suspension Polymerization An aqueous phase (600 ml) consisting of 24 g of acacia gum and 15 g of NaCl is placed in a cylindrical reactor. 1000 ml double glass envelope with vertical baffles and mechanical stirring. The organic phase, consisting of 4-acetoxystyrene (6.5 g, 40 mmol), styrene (6.25 g, 60 mmol), divinylbenzene (13 g, 20 mmol), AIBN (0.34 g) and 2- Ethylhexanol (22.6 g) is then rapidly added. The role of acacia gum is to avoid agglomeration of the balls. The suspension is then stirred at 220 rpm and heated at 80 ^° C. for 15 h. After cooling, the polymer beads obtained are washed abundantly with water and then extracted continuously with soxhlet with water (24 h) then with THF (24 h). They are then dried under vacuum at 40 ^° C. for 24 hours. 23.8 g of white beads (yield: 92.5%) of size between 0.2 and 1 mm are recovered. Different lots are prepared in a similar way. The dimensional characteristics of the different support batches are summarized in Table 1 below.

Diameter Population Level

Mean number Deviation from the trust type lot

(μm) the sample (95.0%)

1 283 115 345 12

2 478 375 165 58

3 486 268 208 37

4,501,541,467 49

5,373 219,388 22

6,418 227,379 23

7,483,369 315 41

8,663 253 154 40

9,397,259,370 26

10,304,208,651 16

11,516 275,286 32

12,398 266,482 24

13,342,237,512 21

14,376 214,349 22

Grafting of the catalyst onto the support Support beads obtained according to the method described above (34.18 g) are placed in a stainless steel column of liquid chromatography type (internal diameter 19 mm, useful length 300 mm). A solution of Ti (OBu) ₄ in p-xylene (50/50 by weight, 21.5 g of Ti (OBu) ₄ , ie 1.5 eq of Ti (OBu) ₄ for 1 eq of OAc functions) is then pumped through the column until complete filling thereof (flow 1 mL / min). The column is then plugged and placed in a heated study at 140 ^° C. for 72 hours. After cooling, a bead sample is taken, in glove box, for elementary analysis of the titanium content.

The characteristics of the catalyst supported on beads obtained are summarized in Table 2 below:

EXAMPLE 2 Test of the Activity of the Supported Catalyst for the Transesterification of Methyl Methacrylate (MMA) with Butanol The reaction studied is the transesterification of methyl methacrylate (MMA) with butanol (BuOH) to form butyl methacrylate ( MBA) and methanol.

The previously described stainless steel column reactor (HPLC type column, 300 mm in height, 19 mm internal diameter) was used in which the supported catalyst was prepared. The flow rate of reagents was set at 0.18 ml / min so as to ensure a residence time of the reactants in the 8 hour column. The feeding of the column is from top to bottom. The output of the reactor is directly connected to a stainless steel coil immersed in an ethanol bath cooled to -20 ^° C. This assembly makes it possible to work under slight overpressure. The column is heated via a heating mantle. The actual temperature reached at the reactor core was measured using a thermocouple (T = 120 ° C). Before the start of the reaction, the column filled with supported catalyst as prepared above, was washed for a long time with the excess of non-titanate. pumped with p-xylene until a colorless eluate was obtained (flow rate 1 ml / min). After stabilization of the column temperature, the pumping of the reagent mixture could begin as well as the fractionation of the eluate and its analysis.

The operating conditions used for the continuous transesterification are summarized in Table 3 below:

The device was kept in continuous operation for a period of 15 days.

The evolution of butanol disappearance rate and formation of butyl methacrylate (MBA) was monitored by Gas Chromatography using an internal standard (undecane) under the following conditions:

- Varian 3300 apparatus equipped with a capillary column of the DB-5 type (I = 30 m, d = 0.25 mm, film thickness = 0.1 μm).

- Temperature gradient from 40 ⁰ C to 220 ⁰ C at 6 ° C / min

(injector 220 ^° C., detector 240 ° C.)

Under these conditions, the retention times are for 4.2 min MMA, 3.6 min BuOH, 10.2 min MBA, and 13.5 min undecane. The evolution over time of the formation rate of butyl methacrylate calculated from these data is reported in Figure 1. A conversion rate greater than 60% is reached after 8 hours of use: the equilibrium of the reaction seems to be reached from this moment. This rate remains relatively stable for 120 hours. The conversion rate remains above 50% for 15 days.

trained tëa $

10B tSQ-200 2SD.

Time of use {hours}

FIG. 1: Evolution of the conversion rate during the continuous activity test of the catalyst supported on beads at 120 ^° C. with a residence time of the reagents of 8 h.

EXAMPLE 3 Test of the Activity of the Supported Catalyst for the Synthesis of Dimethylaminoethanol Acrylate (ADAME) by Transesterification of Ethyl Acrylate (EA) with Dimethylaminoethanol (DMAE) The procedure of Example 2 is reproduced but using 995 g of ethyl acrylate (ie 9.9 moles of EA) and 436 g of dimethylaminoethanol (ie 4.9 moles of DMAE), with 35 g of supported catalyst as prepared in the example 1. The residence time in the column is 8 h at a temperature of 110 ^° C. The device is kept in continuous operation for a period of 5 days, at the end of which an analysis of the reaction mixture is carried out by chromatography. in the gas phase. It is found a conversion of 50% expressed in DMAE reacted after the 5 days of use of the catalyst, which reflects a good stability of the catalyst.

Claims

A process for preparing an alkyl titanate catalyst supported on a polymeric material comprising the steps of: (a) preparing support beads by suspension polymerization of at least two monomers having ethylenic unsaturations, at least one monomer having at least one acetoxyl function,

2. Method according to claim 1, characterized in that the monomer comprising at least one acetoxyl function is 4-acetoxystyrene, N- (p-acetoxyphenyl) maleimide, 3-methoxy-4-acetoxystyrene, dimethoxy-4-acetoxystyrene, 3,5-diacetoxystyrene, 3,4-diacetoxystyrene.

3. Process according to claim 1 or 2, characterized in that styrene, divinylbenzene, trivinylbenzene, divinylethylbenzene, divinylxylene, divinylnaphthalene, divinylchlorobenzene, divinylpyridine, trimethylol trimethacrylate are used as monomers. propane, ethylene glycol diacrylate, divinyl ether diethylene glycol, bisphenol dimethacrylate, pentaerythritol tetra and trimethacrylate, diallyl phthalate, triallyl pentaerythritol, divinyl sulfone, divinyl ketone, divinyl sulfide, diallyl carbonate, 1,2,4-trivinylcyclohexane, 1,5-hexadiene, 1,5,9-decatriene, 1,9-decadiene, 1,5-heptadiene and similar compounds.

4. Method according to any one of the preceding claims, characterized in that step (a) is carried out from a mixture of styrene, divinylbenzene and 4-acetoxystyrene.

5. Process according to any one of the preceding claims, characterized in that the alkyl titanate is chosen from tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, tetrabutyl titanate, tetraisobutyl titanate, tetra 2-ethylhexyl titanate, or else dimethylaminoethyl titanate or titanium phenolate, and mixtures thereof.

6. Method according to any one of the preceding claims, characterized in that the molar ratio titanate / acetoxyl function is between 1 and 4, and preferably between 1, 2 and 2.

7. Process according to any one of the preceding claims, characterized in that step (b) is carried out at a temperature ranging from 50 ^° C. to 150 ^° C.

8. Process for the continuous synthesis of (meth) acrylic esters of formula (I):

in which R has the abovementioned meaning and R2 is a linear or branched alkyl group having from 1 to 3 carbon atoms, with an alcohol of formula

RrOH in which R 1 has the abovementioned meaning, in the presence of a catalyst obtained by a process according to any one of the preceding claims.

9. Process according to claim 8, characterized in that the compound (I) is butyl methacrylate or dimethylaminoethanol acrylate.

10. Use of a catalyst obtained by a process according to any one of claims 1 to 7, for the continuous transesterification of light (meth) acrylates of formula (II):

R 1 -OH (III) in which R 1 is a linear or branched alkyl radical, or a cyclic aliphatic, aryl, alkyl-aryl or aryl-alkyl radical, containing from 4 to 40 carbon atoms, or a linear or branched alkyl radical containing at least one heteroatom and from 3 to 40 carbon atoms.