A carbon with four different groups

Section 2 of 4

Carbon's four bonds, as you have seen, point to the corners of a tetrahedron. Put four different groups on those corners, and the molecule becomes handed.

Butan-⁠2-⁠ol is a four-carbon alcohol with its -⁠OH on carbon 2. Carbon 2 is joined to four different groups: a hydrogen atom, an -⁠OH group, a methyl group (-⁠CH₃) and an ethyl group (-⁠C₂H₅).

Butan-2-ol, C₄H₁₀OA chain of four carbon atoms with an -OH group on the second carbon. That carbon carries four different groups, -H, -OH, -CH₃ and -C₂H₅.HHHHHCCCCHHOHHH
Butan-⁠2-⁠ol, CH₃CH(OH)CH₂CH₃. Carbon 2, with the -⁠OH, is joined to -⁠H, -⁠OH, -⁠CH₃ and -⁠C₂H₅: four different groups.

Now picture butan-⁠2-⁠ol next to its reflection. In the reflection the -⁠OH still comes toward you and the H still points away, and the methyl and ethyl groups have swapped sides. The two molecules are a left and a right hand.

Butan-2-ol and its mirror image Two ball-and-stick models either side of a dashed mirror line. Each has a carbon atom in the middle with four different groups: CH₃ and C₂H₅ above, OH coming toward you and H going away from you. In the right-hand model, the mirror image, CH₃ and C₂H₅ have swapped sides. However you turn them, the two models cannot be laid on top of each other with all four groups matching. CH₃ C₂H₅ OH H CH₃ C₂H₅ OH H mirror
Butan-⁠2-⁠ol (left) and its mirror image (right). In both, the -⁠OH comes toward you and the H points away; the methyl and ethyl groups have swapped sides.

A carbon joined to four different groups is called a stereocentre. A molecule with one stereocentre is chiral and comes in two hands. Swap any two of the four groups and you get the other hand.

How to spot one

  • Look at carbons with four single bonds. A carbon in a double bond has 3 directions in a flat triangle, so it is never a stereocentre.
  • Pass over any carbon carrying two or three hydrogen atoms: two of its groups are the same.
  • For each carbon left, compare its four groups whole, all the way to the end of each chain.

Propan-⁠2-⁠ol, the alcohol in many hand sanitisers, looks a lot like butan-⁠2-⁠ol. Its carbon 2 carries -⁠H, -⁠OH and two -⁠CH₃ groups. With two groups the same, its reflection can be turned to fit exactly on top of the original. Propan-⁠2-⁠ol has no stereocentre, and it has only one form.

Propan-2-ol, C₃H₈OA chain of three carbon atoms with an -OH group on the middle carbon. The middle carbon also carries one hydrogen atom and two identical -CH₃ groups.HHHHCCCHHOHH
Propan-⁠2-⁠ol, CH₃CH(OH)CH₃. Carbon 2 carries two identical -⁠CH₃ groups, so the molecule matches its own mirror image.

Amino acids

You have already met glycine, the simplest amino acid. Its middle carbon carries -⁠NH₂, -⁠COOH and two hydrogen atoms, so glycine matches its mirror image. Alanine has a methyl group in place of one of those hydrogen atoms. Now the middle carbon carries four different groups, and alanine comes in two hands.

Glycine, C₂H₅NO₂The simplest amino acid. One carbon atom carries both an -NH₂ group and a -COOH group.HOHNCCOHHH
Glycine: two hydrogen atoms on the middle carbon, so it has only one form.
Alanine, C₃H₇NO₂An amino acid. One carbon atom carries an -NH₂ group, a -COOH group, a -CH₃ group and a hydrogen atom. This is L-alanine, the form found in proteins.HOHNCCOHHCHHH
Alanine: -⁠H, -⁠NH₂, -⁠COOH and -⁠CH₃ on the middle carbon, a stereocentre. The 3D model shows L-⁠alanine.

Glycine is the only one of the 20 amino acids in your proteins with no hand. For the other 19, living things build their proteins from just one hand, called the L form. So the proteins in your body are handed too, and that matters in the next section.

Quick check

Which of these alcohols has a carbon with four different groups? The number in each name says which carbon carries the -⁠OH.