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Question 18.3: Lactic acid can be isolated from sour milk. Is lactic acid c...

Lactic acid can be isolated from sour milk. Is lactic acid chiral?

\underset{Lactic  acid}{\begin{array}{r c}\underset{|}{\quad OH} \quad \underset{||}{O}\qquad \enspace \> \\ CH_3  –  CH  –  C  –  OH \\ \overset{\pmb{3 \qquad \; \; 2 \qquad \; 1\qquad \quad }}{} \end{array}}

ANALYSIS A molecule is chiral if it contains one C atom bonded to four different groups. Identify any C atoms that meet this condition.

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To find out if lactic acid is chiral, list the groups attached to each carbon atom:

\begin{array}{l} \pmb{Groups  on  carbon  1} & \pmb{Groups  on  carbon  2} & \pmb{Groups  on  carbon  3 \; \; \; } \\ 1.  –  OH & 1.  –  COOH \enspace \qquad \; & 1.  –  CH(OH)COOH \\ 2. = O & 2.  –  OH \qquad \qquad \; & 2.  –  H \qquad \qquad \qquad \; \; \\ 3.  –  CH(OH)CH_3 & 3.  –  H \qquad \qquad \quad & 3.  –  H \qquad \qquad \qquad \; \; \\ & 4.  –  H \qquad \qquad \quad & 4.  –  H \qquad \qquad \qquad \; \; \end{array}

Next, look at the lists to see if any carbon atom is attached to four different groups. Of the three carbons, carbon 2 has four different groups, and lactic acid is therefore chiral.

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