Alkanols (1 Viewer)

[cutemouse]

Hello,

Why does the boiling point in alkanols change as the number of carbon atoms get larger?

 

[clintmyster]

isn't it do with greater dispersion forces therefore greater amount of energy required to break these bonds?

 

[q3thefish]

isn't it do with greater dispersion forces therefore greater amount of energy required to break these bonds?

agreed.

 

[annabackwards]

agreed.

+1

 

[cutemouse]

Okay thanks... Ahh crap I wrote in the test that intemolecular forces and thus boiling pt changes. I wrote that more energy is needed to break bonds then crossed it out!!!! and wrote something a little more general.. I hope I get the marks lol.

 

[micuzzo]

Okay thanks... Ahh crap I wrote in the test that intemolecular forces and thus boiling pt changes. I wrote that more energy is needed to break bonds then crossed it out!!!! and wrote something a little more general.. I hope I get the marks lol.

thats sort of right... but u gotta say the bonds i think... also because alkanols make hydrogenbonds with neighbouring molecules (i think.. cant remember lol)

 

[annabackwards]

thats sort of right... but u gotta say the bonds i think... also because alkanols make hydrogenbonds with neighbouring molecules (i think.. cant remember lol)

Yeah, physical properties IE boiling/melting points are determined by intermolecular bonding. But intermolecular bonding is the general term. Depending on your teacher, you might get it right...

Since the question is regarding alkanols and the number of carbon atoms, it's the increasing value of dispersion forces between the increasing carbon atoms in each molecule that increases the boiling and melting points.

It is NOT because of the hydroxyl (OH) group. This is because each alkanol molecule would only one OH group so increasing the length of the carbon chain would not affect the hydrogen bonding.

Although for Alkanols there is the hydroxyl functional group (OH), you'd only mention it when comparing the boiling/melting points of Alkanols to Alkanes.

 

[cutemouse]

Yeah production of metals is quite hard.

Any study tips on how to remember all these little things?

 

[annabackwards]

Yeah production of metals is quite hard.

Any study tips on how to remember all these little things?

I'd just say any physical properties are influenced by intermolecular forces, so bonding between molecules (hydrogen bonding, dipole dipole forces or dispersion forces) whereas chemical properties (what it reacts/doesn't react with) are affected by whether it has a double/triple bond and whether it's polar/ionic.

It's a bit much to remember but once you get down to doing questions/cram like i did, you'll remember them

 

[clintmyster]

Yeah production of metals is quite hard.

Any study tips on how to remember all these little things?

Thats why you make solid notes =P

 

[study-freak]

Yeah, physical properties IE boiling/melting points are determined by intermolecular bonding. But intermolecular bonding is the general term. Depending on your teacher, you might get it right...

Since the question is regarding alkanols and the number of carbon atoms, it's the increasing value of dispersion forces between the increasing carbon atoms in each molecule that increases the boiling and melting points.

It is NOT because of the hydroxyl (OH) group. This is because each alkanol molecule would only one OH group so increasing the length of the carbon chain would not affect the hydrogen bonding.

Although for Alkanols there is the hydroxyl functional group (OH), you'd only mention it when comparing the boiling/melting points of Alkanols to Alkanes.

+1

And if I was to comment on polar hydroxyl group, I would actually say that intermolecular forces due to the polar bonding decrease in strength as the chain length of alkanols increase. If the chain is long, the electronegativity of oxygen atom in the hydroxyl group is insufficient to make the alkanol very polar.
E.g. Methanol is very polar whereas octanol isn't so much. That's why metanol readily dissolves in water but octanol doesn't.

 

[annabackwards]

+1

And if I was to comment on polar hydroxyl group, I would actually say that intermolecular forces due to the polar bonding decrease in strength as the chain length of alkanols increase. If the chain is long, the electronegativity of oxygen atom in the hydroxyl group is insufficient to make the alkanol very polar.
E.g. Methanol is very polar whereas octanol isn't so much. That's why metanol readily dissolves in water but octanol doesn't.

Ooo good point!

 

[jet]

Though doesnt the dipole exist between the O and H atoms in the hydroxyl group? So then it would more be that the chains are alot longer which makes them much more difficult to dissolve in water?

 

[brenton1987]

And if I was to comment on polar hydroxyl group, I would actually say that intermolecular forces due to the polar bonding decrease in strength as the chain length of alkanols increase. If the chain is long, the electronegativity of oxygen atom in the hydroxyl group is insufficient to make the alkanol very polar.
E.g. Methanol is very polar whereas octanol isn't so much. That's why metanol readily dissolves in water but octanol doesn't.

Though doesnt the dipole exist between the O and H atoms in the hydroxyl group? So then it would more be that the chains are alot longer which makes them much more difficult to dissolve in water?

Atoms only influence other atoms if they are within 3 bond lengths away.
Methanol - the C₁-O bond is polar so the molecule is completely misible in water
Ethanol - the C₁-O and C₂-C₁ bonds are polar so the molecule is completely misible in water
Propan-1-ol - the C₁-O, C₂-C₁ and C₃-C₂ bonds are polar so the molecule is completely misible in water
Butan-1-ol - the C₁-O, C₂-C₁ and C₃-C₂ bonds are polar but the C₄-C₃ bond is not polar so the solubility drops to 9.1 %
Pentan-1-ol - 3 polar bonds and 2 non polar bonds ~2.7 % soluble
Hexan-1-ol - trace soluble
Heptan-1-ol - trace soluble
Octan-1-ol - there are 3 polar bonds and 5 non polar bonds so octan-1-ol has 0 % solubility

This explains why pentan-1-ol only has ~2.7 % solubility while pentan-3-ol is completely miscible.