Write the structure of threonine

Threonine is an amino acid with the structure: $\mathrm{CH_3-CH(OH)-CH(NH_2)-COOH}$

Identify possible chiral centres

A chiral centre is a carbon atom attached to four different groups.

Check each relevant carbon atom

• Carbon-2 (the $\alpha$-carbon): $\mathrm{CH(NH_2)}$ It is attached to:

  • $\mathrm{NH_2}$
  • $\mathrm{H}$
  • $\mathrm{COOH}$
  • $\mathrm{CH(OH)CH_3}$

  These are four different groups, so this carbon is chiral.

• Carbon-3: $\mathrm{CH(OH)}$ It is attached to:

  • $\mathrm{OH}$
  • $\mathrm{H}$
  • $\mathrm{CH_3}$
  • $\mathrm{CH(NH_2)COOH}$

  These are also four different groups, so this carbon is chiral.

• Methyl carbon $\mathrm{CH_3}$ is not chiral.

• Carboxyl carbon is $sp^2$ hybridized, so it is not chiral.

Total number of chiral centres

Therefore, threonine contains: $2$ chiral centres.

Comparison with stored correct answer

Stored correct answer = $2$

Our derived answer also = $2$, so they agree.