The number of chiral centres present in threonine is __________

First, we recall that a chiral centre (also called an asymmetric carbon) is defined as a carbon atom that is attached to four different groups. Because the four substituents are all different, the carbon becomes a centre of optical activity and gives rise to non-superimposable mirror images (enantiomers).

Now, let us write the condensed structural formula of the amino acid threonine. In its open-chain form it is

$$\mathrm{HO{-}CH{-}(CH_3)\,{-}\,CH(NH_2)\,{-}\,COOH}$$

We can see three carbon atoms in the main chain, which we label for clarity as follows:

$$\mathrm{C_1\;-\;C_2\;-\;C_3}$$

where

• $$C_1$$ is the carbon of the carboxylic acid group $$\mathrm{COOH},$$
• $$C_2$$ is the carbon that bears the $$\mathrm{NH_2}$$ group,
• $$C_3$$ is the carbon that is bonded to the $$\mathrm{OH}$$ group and the $$\mathrm{CH_3}$$ group.

We examine each carbon one by one to test whether it is attached to four different groups.

For $$C_1$$ (the carboxyl carbon): the two atoms of the double bond in $$\mathrm{C=O}$$ make the carbon trigonal planar; moreover, $$C_1$$ is part of a double bond and is not attached to four separate substituents. Hence $$C_1$$ cannot be chiral.

For $$C_2$$ (the α-carbon): the four groups attached are

$$\begin{aligned} &1.\; \mathrm{NH_2} \\ &2.\; \mathrm{H} \\ &3.\; \mathrm{COOH} \\ &4.\; \mathrm{CH(OH)CH_3} \end{aligned}$$

All four groups are distinct, so $$C_2$$ is a chiral centre.

For $$C_3$$ (the β-carbon bearing the hydroxyl group): the four groups attached are

$$\begin{aligned} &1.\; \mathrm{OH} \\ &2.\; \mathrm{H} \\ &3.\; \mathrm{CH_3} \\ &4.\; \mathrm{CH(NH_2)COOH} \end{aligned}$$

Again, each of these four substituents is different from the others, so $$C_3$$ is also a chiral centre.

We have now checked every carbon atom in the molecule. Only $$C_2$$ and $$C_3$$ satisfy the condition of having four distinct groups. Therefore,

$$\text{Number of chiral centres} = 2$$

So, the answer is $$2$$.