O-chem: Boiling point

[howui3]

Which of the following compounds has the highest boiling point?

a. CH3CH3 - ethane

b. cyclopentane

c. CH3CH2CH2CH3 - butane

d. CH3CH2CH(CH3)CH3 - 2-methyl butane

e. CH3CH2CH2CH2CH3 - pentane

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[dWiz]

Which of the following compounds has the highest boiling point?

a. CH3CH3 - ethane

b. cyclopentane

c. CH3CH2CH2CH3 - butane

d. CH3CH2CH(CH3)CH3 - 2-methyl butane

e. CH3CH2CH2CH2CH3 - pentane

cyclopentane

 

[howui3]

would you like to explain your reasoning? The more van der Waal forces the higher the boiling point? right?

I thought that the n-pentane (which actually has a higher MW b/c it has extra 2 H) would also have more van der Waal forces?

 

[786mine]

Which of the following compounds has the highest boiling point?

a. CH3CH3 - ethane

b. cyclopentane

c. CH3CH2CH2CH3 - butane

d. CH3CH2CH(CH3)CH3 - 2-methyl butane

e. CH3CH2CH2CH2CH3 - pentane

the longer the chain, the larger are the London dispersion forces (or van der walls). that leaves either cylcopentane and pentane.

the reason why cyclopentane will have higher b.p. then pentane, is because besides having the same length as pentane, the cyclopentane are in ring which has a higher bonding force then stright chain. hope i could explain in plain english.

 

[howui3]

thanks!

 

[waltoncj]

the longer the chain, the larger are the London dispersion forces (or van der walls). that leaves either cylcopentane and pentane.

the reason why cyclopentane will have higher b.p. then pentane, is because besides having the same length as pentane, the cyclopentane are in ring which has a higher bonding force then stright chain. hope i could explain in plain english.

2-methyl butane also has five carbons. Would it have a higher or lower melting point than pentane. I know with boiling point, the more branching there is, the lower the boiling point. Is this the same with melting point? thanks.

 

[isd121]

"Branched molecules have slightly lower BP and MP then their straight-chain isomers (Kaplan page-354)." Branched molecules held together less tightly.

 

[786mine]

"Branched molecules have slightly lower BP and MP then their straight-chain isomers (Kaplan page-354)." Branched molecules held together less tightly.

thanks for posting for me.