Question 12 Question 13 Which one of the following statements is true Which one
ID: 705206 • Letter: Q
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Question 12 Question 13 Which one of the following statements is true Which one of the following statements is true? concerning substitution and elimination in tert-butyl bromide? A) the mechanism generally believed to be available to (CH),CBr are S1 and E1 B) the mechanism generally believed to be available to (CHCBr are S 1, Sy2 and E1 C) the mechanism generally believed to be available to (CHshCBr are S1, Sy2 and E2 D) the mechanism generally believed to be available to (CH)CBr are SN1, E1 and E2 A) CH,CHz-S is both a stronger base and more nucleophilic than CH3CH, o B) CH,CH2 S is a stronger base but less nucleophilic than CH,CH2o C) CH,CHS is a weaker base but is more c than CH,CH2O D) CH CH2S is both a weaker base and less nucleophilic than CH,CH2O Question 14 Question 15 Which one of the solvents below is the most The best combination of reactants for polar protic solvent? Dielectric constants (e) are given in parenthesis. A) acetic acid ( 6) B) water (e78) C) methanol ( 33) D) formic acid ( 58) preparing (CH,),CSCH, : A) (CH ,CCICH,SK B) (CH,)CBrCHSNa C) (CH,hCSK + CH3OH Question 16 Question 17 What combination is the best choice in order to Which one of the following compounds gives prepare 3-chloro-1-iodobutane? A) 1-iodobutane C2 (400C) B) 1,3-dichlorobutane Nal (1 equiv) in acetone C) 1,3-iodobutane NaCI (1 equiv) in acetone D) 3-bromo-1-iodobutane NaCI (1 equiv) in acetone the highest subtitution-to-elimination ratio (most substitution least elimination) on reaction with 2-bromobutane? A) NaOCH, B) NaNH2 C) NaC N D) NaC CHExplanation / Answer
12) The conjugate acids of CH3CH2-S- and CH3CH2-O- are CH3CH2-S-H and CH3CH2-O-H. Since the oxygen (O) atom is smaller than the sulfur (S) atom, hence, the O-H bond is stronger than the S-H bond. Thus, the S-H bond can easily split up to release a proton (hydrogen ion, H+) and hence, CH3CH2-S-H is a stronger acid than CH3CH2-O-H. The basicity and the acidity follow reverse orders, i.e, the stronger acid produces the weaker base and viceversa. Since CH3CH2-S-H is a stronger acid as compared to CH3-CH2-O-H, therefore, CH3-CH2-O- is a stronger base.
The nucleophilicity of a nucleophile depends on the ease with which a nucleophile can donate an electron pair to an electron-deficient carbon. Due to higher electronegativity and smaller size of o atom, the negative charge over O atom is CH3CH2-O- is concentrated, i.e, the O atom has a lower tendency to donate an electron pair. This makes CH3CH2-S- a stronger nucleophile. Thus, the correct statement is (C).
13) It is known that tert butyl bromide does not undergo SN2 reactions. In polar solvents, tert butyl bromide undergoes SN1 reactions via the formation of a carbocation. Tert butyl bromide reacts with strong bases via E1 and E2 pathways to give elimination products. Infact, tert butyl bromide reacts with bases and nucelophiles to give elimination products mostly. Thus, the correct statement is (D).
14) It is known that the higher the dielectric constant of a molecule, the more is the ability of the molecule to stabilize charges. In organic reactions, mainly in E1 or SN1 reactions, the solvent plays an important role by stabilizing the transition state or the carbocation intermediate formed. Thus, a solvent with a high dielectric constant will be able to stabilize the charge on the carbocation and is a stronger solvent. Thus, the higher the dielectric constant, the higher is the polarity of the solvent. Water (? = 78) has the highest dielectric constant and therefore, is the most polar. Thus, (B) is the correct answer.
15) A tert alkyl halide is more susceptible to elimination reaction with a base and therefore, will give an alkene rather than the desired product. Consequently, any combination of a tert alkyl halide with a base is a bad choice for preparing a thio ether. Since option (D) employs a methyl halide (no chances of elimination and/or rearrangement) and a bulky base, hence, (D) is the best choice of reagents for preparing (CH3)3CSCH3.
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