Orgo question

[YA7ES]

Can anyone give me a quick breakdown on how to judge the boiling points of different substances. I ALWAYS run across these questions. If i'm not mistaking it goes:

Alkane < Alkene < Alkyne < First degree alcohols < second degree alcohols < third degree alcohols < aldehydes < ketones < carboxylic acids


Is this correct? or did i just make all of that up? also where do arromatics factor into this? thanks.

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[AZFutureDoc]

The two rules of alkanes are: HIGHER weight, HIGHER boiling point. HIGHER weight, LOWER boiling point.

With oxygen containing compounds, the boiling point is HIGHER than its corresponding hydrocarbon. Next is aldehydes and ketones, and last is carboxyllic acids. When comparing two molecules with little classification in common, higher IM forces means higher BP, Higher weight means higher BP.

 

[spyderracing32]

The two rules of alkanes are: HIGHER weight, HIGHER boiling point. HIGHER weight, LOWER boiling point.

With oxygen containing compounds, the boiling point is HIGHER than its corresponding hydrocarbon. Next is aldehydes and ketones, and last is carboxyllic acids. When comparing two molecules with little classification in common, higher IM forces means higher BP, Higher weight means higher BP.

Wouldn't alcohols have higher BPs than aldehydes and ketones because of H-bonding?

For plane-jane hydrocarbons (alkanes, alkenes, alkynes) BP is weight dependant because a higher molecular weight compound is going to have more electrons and thus greater London dispersion forces can be induced. In terms of structure, the more branched a compound is the lower the BP will because because less surface area is available for London Dispersion forces to act through.

Aldehydes and Ketones are next and don't differ much in BP because the only forces at work are dipole-dipole interactions.

After that are the alcohols where BP is dependent on the accessibility of the OH group for it to participate in H-bonding, and the number of OH groups present in the molecules. Molecular weights are a factor as well because the alkyl chain will still participate in london dispersion interactions.

Carboxyilic acids are last because of the H-bonding, large dipole, and the partial charges attributed to the resonance structures of the molecule (basically has everything going for it in terms of intermolecular interactions).

Don't think of these things in terms of matching hierarchy to functional groups. Think of the intermolecular forces at work and which are greater than the others.

 

[Lukkie]

instead of just memorizing the boiling points, learn why x has a higher bp than y. look up: dipole interactions (the reason larger MW molecules have higher BP), and hydrogenbonding (the reason polar molecules have higher BP)

 

[AZFutureDoc]

Ya, an easy scheme is impossible to come up with and should hopefully be unnecessary. This is usually a question of counting heavy molecules, and then counting hydrogen bonds.