SolutionsOrganic Structures

1. Lactic acid

Lactic acid

Write its complete Lewis structure, with all the unshared electron pairs.
Each oxygen keeps two unshared electron pairs: the two O atoms of the OH groups and the O of the C=O.
Which functional groups does it have? Is the alcohol primary, secondary or tertiary?
Carboxylic acid (–COOH) and alcohol (–OH). The alcohol is secondary: the carbon bearing the OH is bonded to two other carbons.
State the hybridization of each carbon atom.
C1 (COOH): sp²; C2 (CHOH): sp³; C3 (CH₃): sp³.
Does it have a stereocentre? How many stereoisomers exist and how are they related?
Yes: C2, bonded to four different groups (H, OH, CH₃ and COOH). There are two stereoisomers, (R)- and (S)-lactic acid, which are enantiomers: non-superimposable mirror images. A 1:1 mixture of both is a racemate, optically inactive.
Which is the most acidic hydrogen? Write the conjugate base and its resonance forms.
The H of the COOH group (pKa ≈ 3.9). Its conjugate base, the lactate anion, shares the negative charge between the two carboxylate oxygens through two equivalent resonance forms; that is why it is far more acidic than the OH of the alcohol (pKa ≈ 16).
Write a functional isomer of lactic acid (formula C₃H₆O₃).
For example glyceraldehyde (2,3-dihydroxypropanal), HOCH₂–CH(OH)–CHO, with an aldehyde and two alcohols, or dihydroxyacetone (1,3-dihydroxypropan-2-one), HOCH₂–CO–CH₂OH, with a ketone and two alcohols.

Adrenaline

Which functional groups does it have?
Two phenols (the OH groups on the ring), a secondary alcohol, a secondary amine and an aromatic ring.
Point out the stereocentre. Is the molecule chiral?
The stereocentre is the carbon bearing the OH of the side chain, bonded to H, OH, the ring and the CH₂NHCH₃ group. The molecule is chiral; natural adrenaline is the (R) enantiomer.
Which is the most basic atom? Write the equilibrium with water.
The amine nitrogen, whose lone pair is localized in an sp³ orbital and is not delocalized (it is bonded to sp³ carbons): R₂NH + H₂O ⇄ R₂NH₂⁺ + HO⁻.
Which are the most acidic hydrogens? Justify your answer by the stability of the conjugate base.
The H atoms of the phenolic OH groups (pKa ≈ 9-10): in the conjugate base, the phenoxide anion, the negative charge is delocalized over the aromatic ring. The alcoholic OH is much less acidic because its alkoxide has no resonance forms.
Which atom can act as a nucleophile towards an electrophile? Why?
The nitrogen: it has a readily available unshared electron pair (a region of high electron density) that can attack an electron-poor centre.