Things to Note - Topic 17A Chirality

Chemistry

The product of this reaction, 2-hydroxy-2-methylpentanenitrile, has a chiral centre. Explain why a racemic mixture of 2-hydroxy-2-methylpentanenitrile is formed in this reaction.

• ketone is planar about the carbonyl carbon

• so the CN- nucleophile attacks (equally) from above and below / either side (of the plane)

2-bromobutane can react with aqueous hydroxide ions by an SN1 mechanism. Explain why the butan-2-ol produced from a single optical isomer of 2-bromobutane, using this mechanism, is not optically active.

• a racemic mixture is formed (an equimolar mixture of both optical isomers forms)

• intermediate / carbocation is trigonal planar around the reaction site / C+/ opposite sides (of the plane)

• (equal probability of) attack (by nucleophile / hydroxide ions) from either side / above and below / both sides / opposite sides (of the plane)

State why HOOCCH=CHCOOH has E/Z isomers.

• restricted / limited rotation (about the C=C double bond)

• each carbon atom in the double bond is attached to two different groups (of atoms) / to a H (atom) and a COOH group

Explain what is meant by the term stereoisomers

• compounds with the same structural formula

• where the atoms have a different arrangement in space

Explain how, if at all, student' B’s mistake affects the value of the titre.

• because the sodium hydroxide has been diluted

• the titre will be smaller

Explain how, if at all, student C’s use of a wet conical flask affects the value of the titre.

• no effect on the titre

• because the number of moles of sodium hydroxide is unaffected