Structural Elucidation 2

Let's take a look at this question for structure elucidation.

structural elucidation 2 question

There are 5 observations to consider so let's run this through part by part.

Observation 1

structural elucidation 2 observation and deduction 1

P exhibits optical isomerism, hence a chiral carbon is present in P.

P is neutral so cannot contain acidic functional groups like acid and phenol; and basic functional groups like amine.

P undergoes acid base reaction with sodium so an alcohol functional group is present in P.

Observation 2

structural elucidation 2 observation and deduction 2

P (C5H9ON) undergoes reduction with LiAlH4 to form Q (C5H13ON).

By comparing molecular formula, nitrile functional group is present in P, and primary amine is present in Q.

Cummulative Deduction

structural elucidation 2 cummulative deduction 1

So currently we have gathered the following information about P:
- contains chiral C
- has -OH group
- has -CN group on a terminal carbon

Observation 3

structural elucidation 2 observation and deduction 3

Q undergoes oxidation with KMnO4 to form R (C5H11O2N)

By comparing molecular formula, we can deduce a primary alcohol is present in Q and P, and acid group is present in R.

Cummulative Deduction

structural elucidation 2 cummulative deduction 2

So now here are the deductions about P:
- contains chiral C
- has primary -OH group on terminal carbon
- has -CN group on a terminal carbon
- has 5 carbons

Since we have quite a few information about P, we can try to figure out the identify of P.

Remember if we can identify a compound as early as possible, it's much easier to figure out the rest of the observations.

Deduce Chain Isomers

structural elucidation 2 deduce chain isomers for P

For 5 carbon we have 3 possible chain isomers: pentane, 2-methylbutane, 2,2-dimethylpropane

Add Functional Groups and Identify Chiral Carbon

structural elucidation 2 identify P

Notice when primary alcohol and nitrile groups are added to each chain isomer, only 2-methylbutane chain isomer will give us a chiral carbon.

Therefore we have 2 possible structures for P identified.

Once P is identified, we can figure out Q and R:

structural elucidation 2 identify Q and R

Observation 4

structural elucidation 2 observation and deduction 4

Moving on, R undergoes nucleophilic substitution with SOCl2 to form acid chloride S.

So we can identify S since R is already known.

structural elucidation 2 identify S

Observation 5

structural elucidation 2 observation and deduction 5

There is an acid chloride and amine group in S, which can react with each other via condensation to form a neutral cyclic amide in T.

So we can identify both cyclic amides T.

structural elucidation 2 identify T

For the in depth discussion for this exercise, do check this video out!

Topic: Organic Chemistry, A Level Chemistry, Singapore

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