DE1518432C3 - 2-Methoxy-3,6-dichlorobenzyl alcohol and its esters, process for their preparation and their use as pesticides or herbicides - Google Patents

Description

denotes where R has an alkyl or alkylthio group 1 to 18 carbon atoms, a mono-, di- or trichloromethyl group or the group

(H)

X ₅ X ₄

means in which Y is the group

If 20% hydrochloric acid is added to the reaction mixture, an ethereal layer is formed. the Alcohol is then obtained from this layer by known methods, e.g. B. by evaporation in Vacuum on a steam bath. The methyl 2-methoxy-3,6-dichlorobenzoate used as starting material for this is prepared as follows: Dissolve 3,6 dichlorosalicylic acid, a known compound, in acetone and adds potassium carbonate and dimethyl sulfate. The mixture is brought to reflux, cooled and filtered. The clear methyl 2-methoxy-3,6-dichlorobenzoate is obtained by vacuum distillation of the product. Examples of starting materials used in the production of the compounds according to the invention suitable acid chlorides are 2-methyl-4-chlorophenoxyacetyl chloride, dichloroacetyl chloride, 2,4-dichlorophenoxyacetyl chloride, Chloroacetyl chloride, trichloroacetyl chloride, 2 - methoxy - 3,6 - dichlorobenzoyl chloride, 2,4,5 -Trichlorphenoxyacetyl chloride, octadecanoyl chloride, benzoyl chloride, propylthiocarbonyl chloride, Ethylthiocarbonyl chloride, propionyl chloride, phenylthiocarbonyl chloride and methylthiocarbonyl chloride.

The alcohol is mixed with an approximately equimolar amount of acid chloride in benzene, which serves as a solvent. in the presence of a base, e.g. B. pyridine treated. Treatment is by refluxing carried out for a few hours. The product can then be removed from the reaction mixture in a conventional manner be isolated, e.g. By vacuum distillation if the product is a liquid or by recrystallization from a suitable solvent if the product is a solid.

CH ₃

- CHO- or - S-,

35

X ₁ , X ₂ , X ₃ , X ₄ and X ₅ each represent a hydrogen atom, a chlorine atom, a methyl or methoxy radical, and η = 0 or 1.

2. Process for the preparation of esters of 2 - methoxy - 3,6 - dichlorobenzyl alcohol according to Claim 1, characterized in that 2-methoxy-3,6-dichlorobenzyl alcohol is used in a manner known per se with an acid chloride of the general formula

Cl-C-R

wherein R has the meaning given in claim 1, reacts.

3. Use of a compound according to claim 1 as a pesticide or as a herbicide.

In the reaction according to the invention, approximately the same molar ratios of the alcohol are usually used and acid chloride applied.

The new alcohol used as a reactant in the process according to the invention, 2-methoxy-3,6-dichlorobenzyl alcohol, can be expediently carried out by reacting methyl 2 - methoxy - 3,6 - dichlorobenzoate with lithium aluminum hydride in anhydrous ether at low temperatures.

example 1
Manufacture of 2-methoxy-3,6-dichlorobenzyl alcohol

168 g (0.8 mol) of 3,6-dichlorosalicylic acid were in 500 ml of chemically pure acetone dissolved in a 1-1 flask equipped with a condenser and stirrer. Then 235 g (1.70 mol) of potassium carbonate and then 161 ml (1.70 mol) of dimethyl sulfate were added. The mixture was stirred and refluxed for 17 hours, cooled and filtered. The filter cake was washed with acetone and the combined acetone solutions were placed on the steam bath constricted. The residue was distilled in vacuo and gave 168 g (87.5% of theory) of clear Methyl - 2 - methoxy - 3,6 - dichlorobenzoate, boiling point: 115 ° C / 2.4 mm.

131 g (3.44 mol) of lithium aluminum hydride and 21 of anhydrous ether were placed in a 5-liter flask fitted with a stirrer, thermometer, dropping funnel and condenser with an attached drying tube. The slurry was cooled to -70 ° C. in an acetone-dry ice bath and a solution of 809 g (3.4MoI) of methyl 2-methoxy-3,6-dichlorobenzoate in 2 liters of anhydrous ether was added dropwise. After about half of the solution had been added, a very rapid rise in temperature from -70 to 0 ^° C. was observed. The mixture was allowed to cool to -50 ° C before the remainder of the solution was added. The reaction mixture was stirred at -40 to -50 ° C for 3.5 hours and then allowed to warm to -10 ° C. Water was added dropwise until no more gas evolution was observed. 20% hydrochloric acid was added until the mixture split into two

separated clear layers. The ether layer was separated off and the aqueous layer was extracted twice with ether. The combined ether solutions were washed with water, dried over magnesium sulfate, filtered off and concentrated in vacuo on the steam bath. This gave 683 g (95.8% of theory) of colorless 2 - methoxy - 3,6 - dichlorobenzyl, melting point 60 to 67 ° C. A sample was recrystallized from dilute ethanol to give a product of melting point 70.5 to 71.5 C ⁰ recrystallized.

Example 2

a) 2-methyl-4-chlorophenoxyacetyl chloride

In a 1-1 three-necked flask equipped with a stirrer, condenser and dropping funnel were 110.3 g (0.55 mol) of 2-methyl-4-chlorophenoxyacetic acid and 110 ml of benzene were added. This mixture 44 ml (0.61 mol) of thionyl chloride were added with stirring. The reaction mixture was 6 hours heated with stirring and reflux, dried over magnesium sulfate and filtered off. After distilling off of the solvent and excess thionyl chloride obtained under reduced pressure a brown liquid which, after vacuum distillation, 95 g (78.7% of theory) is clear, yellow 2 - methyl - 4 - chlorophenoxyacetyl chloride, boiling point 85 to 10C / 0.05 to 0.12 torr.

b) 2-methoxy-3,6-dichlorobenzyl-2'-methyl-

4'-chlorophenoxyacetate

A 300 ml three-necked flask equipped with a stirrer, condenser and dropping funnel was charged with 10.8 g (0.052 mol) of 2-methoxy-3,6-dichlorobenzyl alcohol in 50 ml of benzene. After adding 4.6 ml (0.057 mol) of pyridine, the mixture was admixed with 11.4 g (0.052 mol) of 2-methyl-4-chlorophenoxyacetyl chloride while stirring. An exothermic reaction occurred on addition and a precipitate of colorless pyridine hydrochloride formed. The reaction mixture was refluxed with stirring for 6.5 hours and then filtered. The filter cake was washed 3 times with benzene. The combined benzene solutions were dried over magnesium sulfate, filtered and concentrated in vacuo. The colorless solid residue was recrystallized from heptane and yielded 13.0 g (64% of theory) of 2-methoxy-3,6-dichlorobenzyl -T- methyl-4'-chlorophenoxyacetate, melting point 75.5 to 75.7 ° C.

Analysis for C ₁₇ H ₁₅ Cl ₃ O ₄ :

Calculated ... C 52.37, H 3.85, Cl 27.34%;
found .... C 52.32, H 4.20, Cl 27.06%.

According to the procedure and with the devices of Examples 1 and 2, numerous other compounds be made according to the invention. In the following examples, the reactants described for the preparation of the named compounds according to the invention were used. The compound 2-methoxy-3,6-dichlorobenzyl alcohol is referred to as compound A for short.

Example 3

The reaction of compound A with dichloroacetyl chloride provided in 46.7% yield 2-methoxy-sjo dichlorbenzyldichloracetat, colorless needles, mp 64.5 ⁰ C (from n-pentane). Analysis for C ₁₀ H ₈ Cl ₄ O ₃ :

Calculated ... C 37.74, H 2.52, Cl 44.65%; found .... C 37.87, H 2.75, Cl 43.96%.

Example 4

The reaction of compound A with 2,4-dichlorophenoxyacetyl chloride gave 2 - methoxy - 3,6 - dichlorobenzyl - 2 ', 4' - dichlorophenoxyacetate in 57% yield, white needles, melting point 91 to 93.5 ° C (from heptane).

Analysis for C ₁₆ H ₁₂ Cl ₄ O ₄ :

Calculated ... C 46.83, H 2.93, Cl 34.63%; found .... C 46.90, H 3.22, Cl 33.99%.

Example 5

Reaction of Compound A with chloroacetyl chloride provided 2-methoxy-3,6 in 58.8% yield - dichlorobenzyl chloroacetate, colorless liquid, boiling point 135 to 138 ° C / 0.6 Torr.

Analysis for C ₁₀ H ₉ Cl ₈ O ₃ :

Calculated ... C 42.33, H 3.17, Cl 37.57%; found .... C 42.80, H 3.89, Cl 37.14%.

Example 6

The reaction of compound A with trichloroacetyl chloride gave 2-methoxy in 65.5% yield - 3,6 - dichlorobenzyl trichloroacetate, light yellow 'liquid, boiling point 150 to 160 ° C / 0.7 Torr.

Analysis for C ₁₀ H ₇ Cl ₅ O ₃ :

Calculated ... C 34.04, H 1.99, Cl 50.35%; found .... C 34.59, H 2.36, Cl 49.46%.

Example 7

The reaction of compound A with 2-methoxy-3,6-dichlorobenzoyl chloride gave 68.2% strength Yield of 2-methoxy-3,6-dichlorobenzyl-2'-methoxy-3 ', 6'-dichlorobenzoate, viscous yellow liquid, boiling point 211 to 215 ° C / 0.5 torr.

Analysis for C ₁₆ H ₁₂ Cl ₄ O ₄ :

Calculated ... C 46.83, H 2.93, Cl 34.63%; found .... C 47.02, H 3.22, Cl 34.25%.

Example 8

The reaction of compound A with 2,4,5-trichlorophenoxyacetyl chloride gave 2 - methoxy - 3,6 - dichlorobenzyl - 2 ', 4', 5 '- trichlorophenoxyacetate in 68.9% yield, colorless solid substance, melting point 127 to 129 ° C (from heptane).

Analysis for C ₁₆ H ₂₂ Cl ₅ O ₄ :

Calculated ... C 43.19, H 2.47, Cl 39.93%; found C 43.75, H 2.94, Cl 39.20%.

B e i s ρ i e 1 9

The reaction of compound A with octadecanoyl chloride gave 2-methoxy in 72.1% yield - 3,6 - dichlorobenzyloctadecanoate colorless liquid, boiling point 220 to 225 ° C / 0.3 Torr.

Analysis for C ₂₄ H ₃₈ Cl ₂ O ₃ :

Calculated ... C 64.71, H 8.60, Cl 15.92%; found .... C 65.30, H 9.06, Cl 15.16%.

Example 10

The reaction of compound A with benzoyl chloride gave 2-methoxy-3,6-dichlorobenzylbenzoate in 73.7% yield, colorless liquid, boiling point 150 to 155 ° C / 0.2 torr.

Analysis for C ₁₅ H ₁₂ Cl ₂ O ₃ :

Calculated ... C 57.90, H 3.89, Cl 22.79%; found .... C 58.29, H 4.04, Cl 22.51%.

Example 11

The reaction of compound A with acetyl chloride gave 2 - methoxy - 3,6 - dichlorobenzyl acetate, colorless Liquid, boiling point 106 ° C / 0.4 Torr.

Analysis for C ₁₀ H ₁₀ Cl ₂ O ₃ :

Calculated ... C 48.21, H 4.05, Cl 28.47%; found .... C 48.17, H 4.22, Cl 29.10%.

Example 12

The reaction of compound A with propylthiocarbonyl chloride gave 2 - methoxy - 3,6 - dichlorobenzylpropylthiocarbonate, colorless liquid, boiling point 128 to 133 ° C / 0.15 torr.

Analysis for C ₁₂ H ₁₄ Cl ₂ O ₃ S:

Calculated ... C 46.61, H 4.56, Cl 22.93, S 10.37%; Found ... C 46.89, H 4.47, Cl 22.93, S 10.81%.

Example 13

The reaction of compound A with ethylthiocarbonyl chloride gave 2 - methoxy - 3,6 - dichlorobenzylethylthiocarbonate, viscous colorless liquid, boiling point 125 to 127 ° C / 0.2 torr.

Analysis for C ₁₁ H ₁₂ Cl ₂ O ₃ S:

Calculated ... C 44.76, H 4.10, Cl 24.02, S 10.86%; Found ... C 44.96, H 4.26, Cl 24.43, S 10.09%.

Example 14

The reaction of compound A with propionyl chloride gave in 72.7% yield 2-methoxy-3,6-dichlorobenzyl propionate, colorless liquid, boiling point 95 to 97 ° C / 0.2 torr. Analysis for C ₁₁ H ₁₂ Cl ₂ O ₃ :

Calculated ... C 50.21, H 4.60%; found .... C 49.99, H 4.64%.

Example 15

The reaction of compound A with phenylthiocarbonyl chloride gave 2 - methoxy - 3,6 - dichlorobenzylphenylthiocarbonate, viscous colorless liquid, boiling point 185 to 190 ° C / 0.3 Torr.

Analysis for C ₁₅ H ₁₂ Cl ₂ O ₃ S:
Calculated ... C 52.49, H 3.52, Cl 20.69, S 9.34%; Found ... C 52.96, H 3.71, Cl 20.50, S 9.90%.

Example 16

Reaction of Compound A with methylthiocarbonyl chloride provided 2-methoxy in 83.6% yield - 3,6 - dichlorobenzylmethylthiocarbonate, viscous colorless liquid, boiling point 120 to 122 ° C / 0.15Jorr.

Analysis for C ₁₀ H ₁₀ Cl ₂ O ₃ S:

Calculated ... C 42.72, H 3.59, Cl 25.22, S 11.40%; Found ... C 42.98, H 3.67, Cl 25.47, S 11.78%.

Example 17

The reaction of compound A with «, 2-dimethyl-

4-chlorophenoxyacetyl chloride gave 2-methoxy-3,6-dichlorobenzyl-u, 2'-dimethyl-4-chlorophenoxyacetate, colorless needles, melting point 77.5 to 79 ^° C. (from hexane) in a 77% yield.

Analysis for C ₁₈ H ₁₇ Cl ₃ O ₄ :

Calculated ... C 53.55, H 4.24, Cl 26.55%;
found .... C 53.76, H 4.47, Cl 26.22%.

The compounds according to the invention are generally used for practical use as herbicides processed into herbicidal compositions that have an inert carrier and a toxic herbicidal Amount of such compounds included. Such herbicidal compositions usually found in referred to in the art as formulations, enable one to obtain the active compound can easily be applied to the weed-infested areas in any desired amount. These compositions can be solid, e.g. B. dusts, granules or wettable powders. You can also be liquids, e.g. B. solutions or emulsifiable Concentrates.

For example, you can create dust by grinding and mixing the active compound with a solid inert carrier, e.g. B. the talc earth, Make clays, silica or pyrophyllite. Granulate formulations can be produced by carriers granulated with the compound usually dissolved in a suitable solvent, z. B. the attapulgites or the vermiculites, usually with a particle size range of about 0.3 to 1.5 mm, impregnated. Wettable powder, which you can in water up to any desired concentration of the active compound can be dispersed by incorporating wetting agents into concentrated Prepare dust compositions.

In some cases the active compounds are in common organic solvents such as Kerosene or xylene are so soluble that they can be used directly as solutions in these solvents can. However, preferred liquid herbicidal compositions are emulsifiable concentrates which an active compound of the invention and, as the inert carrier, a solvent and an emulsifying agent contain. Such emulsifiable concentrates can be used as a spray to the from weed infested areas to any desired concentration of active compound with water be diluted. The emulsifiers most frequently used in these concentrates are nonionic or mixtures of nonionic with other ionic surfactants ».

An example of a herbicidal composition according to the invention is illustrated by the following example explained, in which the amounts are given in parts by weight.

Example 18

Manufacture of a dusting agent

Product from example 2 10

Powdered Talc 90

The specified ingredients are mixed in a mechanical mixing device and

grind until a homogeneous free flowing dust with the desired particle size is obtained. This dust is suitable for direct application to weed areas.

The herbicidally active compounds according to the invention can be used in any known manner. The concentration of the invention Compounds in the herbicidal compositions vary widely with the nature of the formulation and the intended use, ι ο However, the herbicidal compositions generally contain about 0.05 to 95 percent by weight active ingredient. The herbicidal compositions preferably contain about 5 to 75 percent by weight of the active compound. The compositions can also contain other substances, e.g. B. other pesticides. Spreading agents, adhesives, fertilizers, activating agents and dynergistic agents.

Weeds are undesirable plants in their location with no economic benefit and harm Crop crops or the health of livestock. There are numerous Types of weeds known including annuals, e.g. B. Foxtail, white goosefoot, yellow foxtail grass, Finger millet. Mustard. French cabbage, English raised grass, Vogelknötcrich, sand cabbage and water pepper; biennial, e.g. B. wild carrot, burdock, woolly cabbage, round-leaved mallow, mean Adder's head, common thistle, dog's tongue, woolen moth and red thistle knapweed; or perennial, e.g. B. dagger, persistent mole, field mercury, Sudan grass, saudi thistle, hedgehog, Dog tooth grass, field sorrel, field hornwort and barbara herb. Such weeds can also be found classify as broad-leaved or grassy herbs. For economic reasons, it is desirable that the Control the growth of such weeds without harming crops or livestock will.

The compounds according to the invention are particularly valuable for combating weeds, since they are used for many species and genera of weeds are toxic, but proportionally for many crops non-toxic. According to the invention, for weed control, the weeds with a herbicidal composition, which is an inert carrier and an essential active ingredient according to the invention Compound in an amount that is herbicidal and toxic to the weeds, brought into contact. The exact amount of compound required will depend on several factors, z. B. on the resistance of a certain weed species, the weather, the application technology and of the type of crops in the same area. So although the application of up to only about 0.07 or 0.14 kg / ha for effective control of a light infestation with weeds that are under unfavorable Conditions grow, sufficient, however, the application of 1.13 kg / ha or more may apply effective compound for effective control of a strong infestation with resistant weeds, which grow under favorable conditions may be required.

The herbicidal toxicity of the compounds used according to the invention can be broken down by many provide evidence of test methods commonly used in this area. For example, emulsifiable concentrates are diluted with water to concentrations corresponding to 4.5 kg / ha of the active compound. Two at a time Paper pots filled with a mixture of sand and earth are inseminated with weeds. Right away After the insemination, the surface of each pot is thinned with a correspondingly Sprayed test solution. The rearing is carried out under artificial lighting, whereby by setting the porous pots in a small amount of water in stainless steel bowls provided for irrigation will. The weeds are observed for a week to 10 days and the percent kill. Damage and stunted growth are recorded. The results show a high degree of herbicidal effectiveness of the compounds used according to the invention. You can also use appropriate Apply pre-planting and post-emergence tests.

The toxicity of the compounds used according to the invention for weeds can be determined by various factors prove known test methods. In one series of tests, aqueous emulsions are used formulated test substances with different dosages on the surface of the soil, which is less than 24 hours previously with seeds of crops and weeds was sown, sprayed. After spraying the containers were placed in the greenhouse and, if necessary, with additional light and supplied with heat and watered daily or more often. The type and severity of damage in each Crop and each weed was determined 15 to 20 days after the soil was treated. The severity of the damage was rated on a scale from 0 to 10 as follows: 0 - none Damage; 1.2 = slight damage; 3.4 = moderate damage; 5.6 = moderate severe damage; 7.8.9 = severe damage; 10 = death.

Some of the results of these investigations are given in the following tables.

For comparison purposes, the 2,4-dichlorophenoxyacetic acid with examined. This compound, which is abbreviated as 2,4-D in the table below, is the most widely used synthetic organic herbicide (see Ullmanns Encyclopedia der Technischen Chemie, Vol. 15, p. 150, 1964).

Table 1 Conc Damaging Fingergrass (crabgrass) tration ') supply Test substance 4.52 9 9.04 10 Product of example 1 2.26 9 Product of example 1 9.04 10 Product of example 2 2.26 Product of example 2 9.04 K) Product of example 3 2.26 9 Product of example 3 9.04 8th Product of example 4 2.26 7th Product of example 4 April 9 9 Product of Example 7 0 0 Product of Example 7 2.26 6th control the test substance / aniie.uehen. Comparative sample 2.4-D 609 650/11 ') Concentrations are in ki · ha der

Table Il
Coltsfoot (Dock)

Test substance

Product of Example 2 Product of Example 2 Product of Example 3 Product of Example 3 Product of Example 10 Product of Example 10 Product of Example 15 Product of Example 15 Product of Example 16 Product of Example 16 Control
2,4-D (comparison)

Table III
Field mustard (mustard)

Test substance

Product of
Product of
Product of
Product of
Product of
Product of
Product of
Product of
Product of
Product of
Product of
Product of
Product of
Product of
control

Example 1 Example 1 Example 2 Example 2 Example 3 Example 3 Example 7 Example 7 Example 10 Example 10 Example 15 Example 15 Example 16 Example 16

Table IV
Foxtail (pigweed)

Test substance

product
product
product
product
product
product
product
product
product
product

of Example 1 of Example 1 of Example 2 of Example 2 of Example 3 from Example 3 from Example 4 from Example 4 from Example 7 from Example 7

concentration

2.26

9.04

2.26

9.04

2.26

9.04

2.26

9.04

2.26

9.04

2.26

concentration

4.52 9.04 2.26 9.04 2.26 9.04 2.26 9.04 2.26 9.04 2.26 9.04 2.26 9.04 0

concentration

damage

9 10

9 10

9 10 10 10 10 10

damage

9 10 9 9 8 9 9 10 8 9 9 9 0

damage

4.52 6th 9.04 9 2.26 10 9.04 10 2.26 10 9.04 10 2.26 9 9.04 8th 2.26 9 9.04 10

Exam Sulistan /

Product of
Product of
Product of
Product of
Product of
Product of
control

Example 10 Example 10 Example 15 Example 15 Example 16 Example 16

Conc Schüdi tration guni: 2.26 9 9.04 10 2.26 9 9.04 10 2.26 9 9.04 10 0 0

Table V

Grieswurz (Velvet Leaf)

Test substance

Conc Schiidi tration supply 2.26 9 9.04 - 2.26 10 9.04 10 2.26 9 9.04 10 0 0 2.26 6th

Product of Example 2 Product of Example 2 Product of Example 10 Product of Example 10 Product of Example 16 Product of Example 16 control

2,4-D (comparison)

In a further series of tests, the one used in accordance with the invention to demonstrate the toxicity Compounds was carried out, the test substances formulated as aqueous emulsions with different dosages on the leaves of crops and weeds that have a predetermined size had reached, sprayed on. After spraying, the plants were placed in a greenhouse and watered daily or more often. No water was applied to the leaves of the treated plants. The type and severity of damage for each crop and each weed was 13 days determined after the treatment of the plants. The degree of damage was on the same scale as rated in the previous row. Some results of these investigations are in the following tables specified:

Table VI

Coltsfoot (Dock)

Test substance

concentration

damage

Product of
Product of
Product of
Product of
Product of
Product of
Product of
Product of
Product of
control

Example 2 Example 2 Example 3 Example 3 Example 4 Example 7 Example 7 Example 16 Example 16

2.26 8th 9.04 10 2.26 9 9.04 10 2.26 9 2.26 9 9.04 9 2.26 7th 9.04 10 0 0

11th

Table VII Field mustard

I test sub-plan /

Product of Product of Product of Product of Product of Product of Product of product of control

Example 2 Example 2 Example 4 Example 4 Example 7 Example 7 Example example

Table VIII Pigweed

Test substance

concentration

2.26 9.04 2.26 9.04 2.26 9.04 2.26 9.04 0

concentration

Damage:

9 10 10 10

damage

Product of example 2 2.26 10 Product of example 2 9.04 10 Product of example 3 2.26 7th Product of example 3 9.04 10 Product of example 4 2.26 8th

I am submissive

Conc Schiidi tration supply 9.04 9 2.26 9 9.04 10 0 0

5. Product of Example 4
Product of Example 7
Product of Example 7
control

ίο The results of both series of tests were particularly surprising in view of the fact that the test substances were low in practically all cases or exhibit no toxicity to maize and wheat at dosages as high as 9.04 kg / ha.

The compounds are therefore particularly useful for the selective weed control of such crops Importance.

In addition, 2-methoxy-3,6-dichlorobenzyl alcohol was tested as a systemic aphicide. The compound was dissolved in acetone and then _{dispersed in water at a concentration of 100} %. Plants infested with adult pea lice were pulled out together with the roots, freed from soil by washing and placed in a glass vessel containing 100 ml of the dispersion of the test substance. Aluminum foil was attached around the plant stems and the vessel in order to reduce the possible escape of toxic fumes from the dispersion to be tested. The test plants were then kept in the greenhouse for 72 hours and observed for systemic aphicidal activity. The test was repeated three times. The percent mortality was 90%.

Claims (1)

Patent claims: 1. 2 - Methoxy - 3,6 - dichlorobenzyl alcohol and its esters of the general formula Cl H Cl in which M is a hydrogen atom or the group O Il -C-R