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A fruity ester that rotates light

Deducing structures from reactions and data; chirality through a reaction; ester synthesis and safety

Compound **X**,

\ce{C5H10O2}

, has a fruity smell and rotates plane-polarised light. IR: 1720 and 1180 cm

^{-1}

; no broad peak between 2500 and 3550 cm

^{-1}

. **X** was heated under reflux with NaOH(aq) and the mixture acidified, giving **A** and **B**. **A**,

\ce{CH2O2}

, fizzes with

\ce{NaHCO3(aq)}

and also gives a silver mirror with Tollens' reagent. **B**,

\ce{C4H10O}

, is optically active and is oxidised by acidified dichromate to **C**,

\ce{C4H8O}

, which does not react with Tollens' reagent and shows four ¹³C peaks.

Facts you may need

ester hydrolysis
oxidation of alcohols
Tollens' test
chirality
¹³C shift ranges

(a)

Identify **A**, **B**, **C** and **X**, giving structures and names. Justify each identification.

(b)

Write equations for the hydrolysis of **X** by NaOH(aq), the acidification step, and the reaction of **A** with

\ce{NaHCO3}

. Explain why reflux is used for the hydrolysis.

(c)

Explain why **A** gives a silver mirror with Tollens' reagent, while ethanoic acid does not.

(d)

State whether **B** and **C** are optically active, and explain why **B** has the same configuration at its chiral carbon as **X**.

(e)

Predict the number of ¹³C peaks for **X**, and assign the expected chemical-shift range for each carbon.

(f)

Discuss how **X** could be prepared from **A** and **B** in the laboratory, including conditions, how to obtain a reasonable yield, and two safety precautions.

(g)

A student reacted 5.00 g of **B** with excess methanoic acid and obtained 4.20 g of pure **X**. Calculate the percentage yield and give two reasons, other than the position of equilibrium, why it is below 100%.