Question

The following molecular models are representations of (a) adenine and (b) cytosine, constituents of DNA (deoxyribonucleic acid). Indicate the positions of multiple bonds and lone pairs for both, and draw skeletal structures (gray 5 C, pink 5 O, blue 5 N, ivory 5 H).

Short answer

Positions 1, 2, 3, 4, 5, 6, 7, 8 contains multiple bonds. In the given structure of adenine, all the nitrogen atoms contain a single lone pair of electrons. So there are a total of 4 lone pairs of electrons.

Step-by-step solution

Multiple bonds

Multiple bonds are chemical bonds that are formed by sharing two or more electron pairs.

Multiple bonds indicate either double or triple bonds. If a double bond is present then one bond is sigma ($\sigma$) and the other is pi ($\pi$). If a triple bond is present one bond is $\sigma$and the other two bonds are $\pi$-bonds.

Position of multiple bonds

In adenine, there are a total of 8 positions that contain multiple bonds.

Lone pair

The lone pair is nothing but the unshared pair of electrons.

In an adenine structure, there are a total of 4 lone pairs. Each nitrogen atom in adenine contains single lone pair of electrons.

Skeletal Structure

Skeletal structure in organic chemistry is a series that shows the series of atoms bonded together to form the structure of a compound.

In simple words, the skeletal structure includes only bonds and atoms other than carbon and hydrogen. The skeletal structure of adenine is as follow;

b) Cytosine

All six positions in cytosine contain multiple bonds. In the given structure of cytosine, all the three nitrogen atoms contain a lone pair and an oxygen atom contain 2 lone pairs of electron so there are a total of 5 lone pair of electrons.