Question

Passage 11 (Questions 1-9) Dienes, a special group of alkenes, can be classified as conjugated or isolated. Conjugated dienes are alkenes with alternating double and single bonds. Isolated dienes have double bonds separated by more than one single bond. One explanation for this observation is the hybridization theory. According to the hybridization theory, the stability of conjugated dienes comes from the hybridization of the orbitals forming the carbon-carbon single bonds. For example, in 1,4-pentadiene, the C-C single bonds result from the overlap of an sp' orbital from one carbon with an sporbital from the adjacent carbon. In conjugated dienes, on the other hand, the CC single bonds result from overlap of sp orbitals on both carbons. conjugated diene isolated diene 1. In a manner similar to other alkenes, conjugated dienes typically undergo electrophilic addition. Which of the following most accurately represents the reaction intermediate formed when 1.3-butadiene reacts with HCI? Although conjugated dienes are similar in reactivity to isolated dienes, conjugated dienes are somewhat more stable. The heats of hydrogenation were measured for several alkenes and dienes to determine their stability (Table 1). The data show that alkenes become more stable with increasing substitution and increasing number of double bonds. For example, in Table I the AH of 1,4-pentadiene, an isolated diene, is 60.8 kcal/mol, approximately double the AH., of 1-butene. However, 1,3-butadiene has a AH of 57.1 kcal/mol. Based on these results, conjugated dienes are considered more stable because they release less heat on hydrogenation 8 8° A. H2 CH2=CH-CH, 8* 8* B. HC-CH-CH---CH2 8 C H,ề-CH=CH-CH, D. HC-CH=CH-CH, Alkene AH (kcal/mol) 30.3 26.9 CH,CH2CH=CH2 + H2 1-Butene CH, CH,CH.C=CH2 + H2 2-Methyl-1-butene H2C=CHCH=CH2 + H2 1,3-Butadiene 2. The carbon-carbon single bond in 1,3-butadiene has a bond length that is shorter than a carbon-carbon single bond in an alkane. This is a result of the A. overlap of two sp orbitals. partial double-bond character due to the o electrons. C. overlap of one spand one sp' orbital. D. overlap of two sp orbitals. 26.7 57.1 H2C=CHCH=CH2 + 2 H2 1.3-Butadiene H,C=CHCH.CH=CH2 + 2Hz 1,4 - Pentadiene 3. In comparison to a normal alkane bond, the C(2)-C(3) bond in 1.3-butadiene has: A. more s character. B. less character. C. more p character. D. less energy 60.8 Table 1 Heats of Hydrogenation of Alkenes 4. What is the IUPAC name of the major product that is formed when 1.3-butadiene is reacted with an equal amount of HCI? A. 1-Chloro-2-butadiene B. 3-Chloro-1-butadiene C. 1-Chloro-2-butene D. 3.Chloro-1-butene 1100

Answer 1

ch or "CH ch₂ 1,3-butadiene ch=ch-cn=c12 CH₂ =cn - ch-CH₂ CH, -ch=ch - CH₂ Intermediates st Overall intermediate :- st си, option ® correct CH "in - C1₂ gn by al lane (-( bond is formed is formed sp-sp overlap. In 1, 3 - butadiene (-( bond is formed by sp²-sp² overlap. Due to more s character (-( bond length in 1,3 - butadiene is shooter. correct option A Correct option ④ 3 HLI प + M Cl (major) (minor) Name of major product option Conect 3-chloro-1- butene