Question

Table 3. Compounds and Unmatched Boiling Points 78°C Compounds Ethanol Dimethyl ether -48 °C Propene -24°C Table 4. Compounds and Unmatched Boiling Points Compounds 108°C 1-Butanol 98-100°C 2-Butanol 2-Methyl-1-Propanol (117.4°C

F. Based on their structures and the IMFs present can you rationalize why these compounds vary in their boiling points?

Answer 1

(a) Between ethanol, dimethyl ether and propene the boiling point order is ehanol (78°c)> dimethyl ether (-24°c)>propene(-48°c)

This can be explained by considering Intermolecular force of attractio. In ethanol CH3-CH2-OH the IMFs is strong hydrogen bonding.In dimethyl ether CH3–O–CH3 which is polar molecule.So IMFs is weak Dipole –Dipole force of attraction.In propene the IMFs is weakest force of attraction.This is because propene is nonpolar hence the IMFs is weakest London force (induced Dipole –induced dipole) force of attraction.

(b) Between 1-butanol,2-butanol and 2-methyl-1-propanol the boiling point order is 1-butanol(117.4°c)> 2-methyl-1-propanol(108°c)>2-butanol(100°c)

This can be explained by considering IMFs in the molecule.In all the above alcohol H- bonding is present but three alcohols are structural isomers of each other. We know with branching surface area decrease hence vanderwaal force of attraction decreases. Primary alcohol(1-butanol,2-methyl-1-propanol)hydrogen bonding is more effective than secondary alcohol(2-butanol).Hence IMFs order is 1-butanol> 2-methyl-1-propanol> 2-butanol