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Question Video: Identifying the General Formula of Esters Chemistry

R is used in organic chemistry to denote a hydrocarbon chain. Which of the following is the general formula of esters? [A] ROR′ [B] RCOOH [C] ROOR′ [D] RCOOR′ [E] ROH

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Video Transcript

R is used in organic chemistry to denote a hydrocarbon chain. Which of the following is the general formula of esters? (A) ROR dash, (B) RCOOH, (C) ROOR dash, (D) RCOOR dash, or (E) ROH.

The question asks about esters. Esters are a broad group of organic compounds which contain a carbon atom double-bonded to an oxygen atom and single-bonded to another oxygen atom. When the atom bonded to this oxygen atom is not a hydrogen atom, but a carbon atom, then we call this functional group an ester group. On either side of this ester group, we have an R and an R dash group. R and R dash are usually used to denote straight alkyl chains such as CH3, the methyl group, or CH2CH3, the ethyl group, or branched alkyl chains, such as the isopropyl group, or even aryl groups, which are groups containing aromatic hydrocarbons rings, such as the benzyl group. R dash cannot be a hydrogen atom as this would make the molecule a carboxylic acid, but R can be a hydrogen atom.

What we have discussed so far is what we were told in the question, that R is used to denote a hydrocarbon chain, or a hydrogen atom over here. Esters can be made by reacting a carboxylic acid with an alcohol in the presence of a dehydrating agent. This portion of an ester comes from the carboxylic acid, and this portion from the alcohol. The carbon-oxygen single bond is called an ester bond. Water is a by-product of this reaction. We can write the general formula of an ester like this, or like this, which shows more correct bond angles, or we can condense it even further. To begin condensing we remove all the single bonds. This partially condensed formula still shows us that this R group is bonded to this carbon atom. We know this because they are next to each other in the condensed formula.

In the same way, this carbon atom and this oxygen atom are bonded, and this oxygen atom to the R dash group. But how do we deal with the carbon-oxygen double bond? We want to condense this portion of the molecule even further. What we do is we remove the double bond and write the oxygen atom in between the carbon atom and the other oxygen atom, which gives us this formula. Remember, there is a double bond between these atoms and a single bond between these two atoms, although we cannot see this from this condensed formula. We need to recognize that these molecule portions are the same. So, which is the general formula of esters? The answer is (D) RCOOR dash.

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