Karplus relationships are based on theoretical calculations of the effect of the
ID: 931457 • Letter: K
Question
Karplus relationships are based on theoretical calculations of the effect of the dihedral angle (torsional angle) between coupling partners on their local magnetic fields. An acid-base reaction is favourable when the base used generates a more weakly basic product. The direct alkylation of Grignard reagents with alkyl halides does not work well. In enolate alkylation, reaction at oxygen is favoured by hydrogen-bonding solvents like HOtBu. In enolate acylation, such as in Claisen condensations, reaction at oxygen is not problematic because the O-acylated product itself serves as an acylating agent. In competing reactions, higher temperatures give better selectivities. The S_NAr mechanism amounts to a conjugate addition-elimination sequence. Because they are less reactive than acid chlorides, acyl imidazoles are less selective acylating agents. Dialkyl ketones are more electrophilic than alkyl aldehydes. The E1_cb mechanism is used during b-elimination under basic conditions.Explanation / Answer
(a) True.
Because Karplus equation explains the relation between coupling constant and dihedral angle in nuclear magnetic resonance spectroscopy.
(b) True
An acid-base reaction is favoured when the acid produce weak conjugate base and base produce weak conjugate acid.
(c) True
Grignard reagent is prepared by alkylation of magnesium metal. So, alkylation of Grignard reagents with alkyl halides does not work well.
(d) False
If oxygen is involve hydrogen bonding with solvent, it nucleophilic strength is decreases. So, alkylation does not favor.
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