Organic Conversions: A Method for Getting From A to B

The two questions to ask first

1. Has the carbon count changed? If yes, a chain-lengthening or shortening step is required somewhere, and identifying it usually solves most of the problem.
2. Which functional group has changed into which? If the carbon count is the same, the whole conversion is functional group interconversion.

Reactions that change the carbon count

These few reactions cover nearly every carbon-count change asked at this level. Learning them as a group, organised by the change they produce, is far more efficient than meeting them scattered through chapters.

The functional group map

For same-carbon conversions, the useful mental object is a map of which group converts to which.

Moving right is oxidation; moving left is reduction. Halides sit centrally because they are the usual gateway into substitution chemistry, which is why so many routes pass through one.

Alkenes connect to this map through addition and elimination, and they are frequently the intermediate that makes an otherwise impossible conversion work — for instance moving a functional group from one carbon to the adjacent one by eliminating and then re-adding with the opposite orientation.

Working backwards

Ask what could have produced the target in one step, then what could have produced that. This is usually faster than working forwards, because the target constrains the possibilities more tightly than the starting material does.

The reasoning is the same as retrosynthetic analysis at a higher level, and starting the habit here makes that topic straightforward later.

Points that cost marks

• Write reagents and conditions, not just arrows. Conditions frequently determine the product, and marks are allocated to them.
• Show intermediates. A multi-step conversion written as a single arrow scores poorly even when the reagents are right.
• Check the position of the functional group, not merely its identity. Markovnikov versus anti-Markovnikov matters.
• Watch for reagents that would attack something else in the molecule.

Worked reasoning example

To convert a halide to an acid with one more carbon: the carbon count rises by one, which immediately points to the cyanide route or the Grignard-with-carbon-dioxide route. Both work; the cyanide route substitutes then hydrolyses, the Grignard route forms the reagent then carboxylates.

Choosing between them depends on what else is present. If the molecule has an acidic hydrogen anywhere, the Grignard route fails and the cyanide route is the answer — and saying why is worth more than the route itself.

Frequently asked questions

How do I add exactly one carbon?

Through a nitrile, or through a Grignard reagent reacting with carbon dioxide. Both add precisely one carbon, and which to use depends on the other groups present.

How do I remove one carbon?

By the Hofmann bromamide reaction from an amide, or by decarboxylation of a carboxylate salt.

Should I show every intermediate?

Yes. Marks are given for intermediates and for reagents at each stage, so a compressed answer loses them even if the endpoints are right.

What if I cannot find a route?

Work backwards from the target one step at a time, and check whether an alkene intermediate helps — converting to an alkene and back is often the way to move a functional group between adjacent carbons.