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Please explain the name of 3-hydroxybutanoic according to IUPAC nomenclature rules

Edits below:

Hi all,

I have a chemistry question in a practise booklet that I believe requires knowledge about IUPAC naming conventions. For the sake of context, I will include the booklet questions and answer choices, but my actual question is about naming conventions. The words in the square brackets are my own reasoning, the rest of the words are from the question itself, bold words are my emphasis.

--- Booklet Question and info starts below: ---

Organic acids that have a hydroxy group and a carboxylic acid group attached to two adjacent carbon atoms are often called by their colloquial name ß-hydroxyacids. This is [it is called beta] because the hydroxy group is on the second carbon atom from the acid group. [edit: I realised why it's called 'beta', see below for explanation]. An example is mevalonic acid, HOCH₂-CH₂-COH(CH₃)-CH₂-COOH, which is a precursor to the production of cholesterol in the body. One method of producing ß-hydroxyacids in the laboratory is to react an aldehyde or ketone with an α−bromo ester (the bromine is attached to the same carbon atom as the carboxylate group) and metallic zinc in ether. Subsequent hydrolysis produces the ß-hydroxyacid. Figure 1 shows the steps in this reaction.

Blue 17 Fig 1.jpg

Note that:
1) the bromo-compound must be an α-bromo alkyl or aryl ester, not an α-bromocarboxylic acid.
2) R', R" and R"' can each be any of H, an alkyl or an aryl group.

Question:
Which one of the following could be produced by the reaction sequence described in Figure 1?
A) 4-hydroxybutanoic acid
B) 3-hydroxy-3-methylpentanoic acid (this is the correct answer)
C) 4-hydroxy-3-methylpentanoic acid
D) 4-hydroxy-4-methylhexanoic acid

--- Booklet Question and info finished ---

EDIT:

I realised that the "beta" in the "colloquial name ß-hydroxyacids" is because, as the information said, it is "the hydroxy group is on the second carbon atom from the acid group." Alpha carbons are the first carbon away from the carbon of interest and the beta carbon is the second carbon away from interest, the alpha and beta (and gamma) naming method is just a naming method to keep track of carbons. Just putting it here in case it's useful to someone.

My questions:
1) According to the IUPAC nomenclature rules, when numbering a parent carbon chain, you should number the carbons so that the substituents have the lowest number possible.

For 3-hydroxybutanoic acid which is shown in this diagram:

3-hydroxybutanoic acid.jpg

There must be 4 carbons since this is a butanoic acid. There are already 3, I just need to allocate another C to one of R', R", and/or R"'. A CH₃ has to be allocated to at least one of R" or R"' because that would allow a chain of 4 carbons (if allocated to both, one will be a side chain). If say R' was a carbon, there would not be a chain of 4 carbons.

Since it is a "3-hydroxy", it means the Hydroxyl group (OH) is on the 3rd carbon.

BUT:
As rule number 2 above, where R" is, if it were a CH_3,according to the IUPAC rules, shouldn't you number the carbon on the R" as carbon 1, giving OH a lower number of 2 so it is 2-hydroxy and not 3-hydroxy?

Is the carbon in the acid group (the COOH) itself, counted as one of the carbons? If so, is this where the OH group gets the the "3-hydroxy" part of the name, since it is on the 3rd carbon counting from the COOH acid end?

(Unless I misunderstood something) The carbon that the OH is on can be either numbered as 2 or 3 depending on which end you start counting from first. If you count from the R" end (including the carbon on the R" itself), it is the second carbon. If you count from the COOH end (including the carbon on the COOH itself), it is the third carbon. At the same time it says it is called "Beta" colloquially because it is on the second carbon from the acid. To me this means the question wants you to number it from the COOH end, and, including the carbon on the COOH would make it the third carbon, which violates the IUPAC rule of numbering it from the end to give it the lowest number possible (which is 2).

For the question asked in the booklet, I can tell the answer is C only because it is the only choice that starts with a "3-hydroxy". My reasoning could be wrong but: If R" and R"' are both CH₃, then one of them forms part of the 4-carbon chain and the other one, connected to the same C as the OH is connected to, would be a "side chain containing 1 carbon", hence the name "methyl". But if one of the answer choices were "2-hydroxy-2-methylpentanoic acid" then I would not know whether the answer should be "3-hydroxy-3-methylpentanoic acid" or "2-hydroxy-2-methylpentanoic acid".

But now I have a new problem: according to the IUPAC rules, the suffix tells you how many carbons are in the straight chain. Answer choice C is "3-hydroxy-3-methylpentanoic acid" with pent meaning 5, but you cannot get a straight 5-carbon chain even if both R" and R"' are carbons, because it is not a continuous straight chain, one of them would be a side chain.

So basically I am very confused about the naming and what is happening here.

Thanks in advance

(edited 1 year ago)

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