Q 43 E - Organic Chemistry (Mcmurry)

Question

Question: Propose structures for compounds that meet the following descriptions:

1. An optically active compound ${\mathbf{C}}_{\mathbf{5}}{\mathbf{H}}_{\mathbf{10}}\mathbf{O}$ with an IR absorption at $\mathbf{1730}\mathbf{ }{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$  .

2.  A non-optically active compound ${\mathbf{C}}_{\mathbf{5}}{\mathbf{H}}_{\mathbf{9}}\mathbf{N}$  with an IR absorption at $\mathbf{2215}\mathbf{ }{\mathbf{cm}}^{\mathbf{-}\mathbf{1}}$.

Step-by-Step Solution

Answer

a)

b)

Step 1: IR spectroscopy

All the absorptions seen in alkanes is due to the C-C and C-H bonds. The C-H bonds show absorption in the region 2850 to $2950 {\mathrm{cm}}^{-1}$ and the C-C bonds are seen in the region 800 to $1300 {\mathrm{cm}}^{-1}$ .Most of the organic compounds exhibit these absorptions as they comprise saturated alkane portion.

Step 2: Proposing the structure of the compound with different description

(a)The optically active compound with the formula ${\mathrm{C}}_5{\mathrm{H}}_{10}\mathrm{O}$ that absorbs at $1730 {\mathrm{cm}}^{-1}$ can be given as:

The IR absorption at this region is due to the saturated aldehyde. The name of the compound is 2-methylbutanal.

(b)The non optically active compound has a formula ${\mathrm{C}}_5{\mathrm{H}}_9\mathrm{N}$  that absorbs at$2215 {\mathrm{cm}}^{-1}$ can be given as:

The absorption at this frequency is due to the cyanide functional group. The given compound is trimethylacetonitrile.