Without referring to tables, decide which member of each of the following pairs

Question

Without referring to tables, decide which member of each of the following pairs would have the higher boiling point. Explain answers. (a) Pentane or 2-methylbutane (b) Heptane or pentane (c) Propane or 2-chloropropane (d) Butane or I-propanol (e) Butane or CH_3COCH_3 One compound whose molecular formula is C_4H_6 is a bicyclic compound. Another compound with the same formula his red absorption at roughly 2250 cm^-1 (the bicyclic compound docs not). Draw structures for each of these two compounds and how the IR absorption allows them to be differentiated. Which compound would you expect to be the more stable: cis-1, 2-dimcthylcyclopropanc or trans-1, 2-dimethylcycloprop. Explain your answer. Consider that cyclobutane exhibits a puckered geometry. Judge the relative stabilities of the 1, 2-disubstitutcd cyclobutane the 1, 3-disubstituted cyclobutanes. (You may find it helpful to build handheld molecular models of representative compounds) Write the two chair conformations of each of the following and in each part designate which conformation would be the stable: (a) cis-1-tert-butyl-3-methylcyclohcxane, (b) trans-1-tert-butyl-3-methylcyclohexane. (c) trans-1-tert-butyl-4-methylcyclohexane (d) cis-1-tirt-butyl-4-methylcyclohexane.

Explanation / Answer

Ans 4.39

a) Pentane

Pentane has higher BP because of the larger surface area as compared to 2-methylbutane which increases the vander vaal forces .

d) Propanol

The propanol has intermolecular hydrogen bonding due to the presence of hydroxyl group . The butane molecule lacks such bonding . Hence BP of propanol is higher.

b) heptane

Heptane has higher BP than pentane due to larger surface area and hence more amount of vander vaal forces as compared to pentane .

e) Propanone (CH3COCH3)

Propanone has higher BP than butane because of the presence of stronger dipole dipole intermolecular attractions as compared to weaker dispersion forces as present in butane .

c) 2-chloropropane

It is has higher molecular mass than propane , hence more amount of energy will be needed to break the intermolecular bonds , hence 2-chloropropane has higher BP than propane.

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