Which bond in the compound to below would require the least energy to undergo ho
ID: 515823 • Letter: W
Question
Which bond in the compound to below would require the least energy to undergo homolytic cleavage? a. Bond A b Bond B c. Bond C d. Bond D e. Bond E Which of the protons in ethyl 3-phenyl-3-oxopropanoate are the most acidic? a) 1 b) 2 c) 3 d) 4 e) 2 & 3 equally Which of the following is an example of a radical abstraction reaction CH*_3 = Br_2 rightarrow CH_3Br + Br* d. All are radical abstraction reactions. e. None are radical abstraction reactions. Draw the major organic product of the following reaction.Explanation / Answer
Answer to Q4 is option A. For the C-H bonds, increase in substitution on the C-atom causes decrease in bond dissociation energies. The bond dissociation energy is the minimum energy required to break a bond into two radicals. The bond dissociation energy follows following order and hence it is easier to break C-H bond of tertiary carbon into two radicals than C-H bond of primary carbon into two radicals.
primary C-H > secondary C-H > tertiary C-H
i.e. H3C• > R-CH2C•H-CH3 > (CH3)3C• and the C-H bond dissociation energies for these bonds are 429 kJ mol-1, 396 kJ mol-1 and 384 kJ mol-1, respectively.
Answer to Q4 is option B. For the alpha hydrogen atoms in a carbonyl group are acidic protons and can be easily abstracted by a base to give carbanion. The acidity of these protons depends on the stability of the carbanion formed i.e. the alpha hydrogen atom is more acidic if the carbanion formed is more stable. The carbanion can be stabilised by resonance or by attaching an electron withdrawing group to the alpha carbon atom. The given structure is of a beta-ketoester. The alpha hydrogen atoms of the ketoesters are more acidic compared to those of the corresponding ketone or ester due to an additional carbonyl group providing it resonance stabilisation.
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