hello all,

Just have a question about melting points in alkanes. Especially about branching. Kaplan blue book states that branchings in alkanes cause lower melting point due to the fact that they can't be tightly packed together(P.286). Destroyer says that branching allows symmetry in the molecule, leading to higher melting point (Question 152). So which is true? :confused:

It would also help if someone can clarify how the length of the chain affects both boiling point and melting point.

Thank you so much!

hello all,

Just have a question about melting points in alkanes. Especially about branching. Kaplan blue book states that branchings in alkanes cause lower melting point due to the fact that they can't be tightly packed together(P.286). Destroyer says that branching allows symmetry in the molecule, leading to higher melting point (Question 152). So which is true? :confused:

It would also help if someone can clarify how the length of the chain affects both boiling point and melting point.

Thank you so much!
I think the longer the chain is, the higher the boiling point.
The more braching, the higher melting point.

ya i have trouble with this too....someone correct me if im wrong

longer chain has higher BP AND MP then shorter

branchine has higher MP but lower BP then unbranched?

double and triple bonds have highler BP then single bonds but single bonds have higher MP? im getting this from the concept of satuarted and unsaturated fat...some1 telle me if this is wrong

Cis has higher BP bc of surface area?
Trans has higher MP bc of symmertry

while we're at it bond lenght ->double> triple
bond order triple>double>single

triple stronger than double stronger thans single.........triple bonds have less intermoluclar forces than double than single right?

if some1 can think of more things add it on

i Think The Longer The Chain Is, The Higher The Boiling Point.
The More Braching, The Higher Melting Point.
+1

ya i have trouble with this too....someone correct me if im wrong

longer chain has higher BP AND MP then shorter

branchine has higher MP but lower BP then unbranched?

double and triple bonds have highler BP then single bonds but single bonds have higher MP? im getting this from the concept of satuarted and unsaturated fat...some1 telle me if this is wrong

Cis has higher BP bc of surface area?
Trans has higher MP bc of symmertry

if some1 can think of more things add it on
Unbranched his higher BP due to greater surface area, which translates to stronger london dispersion forces.

The more branched an alkane, the more compact it becomes, and the better it can be packed into a crystal lattice.


For aklenes, cis isomers have higher boiling points due to stronger intermolecular (dipole-dipole interactions) forces. Cis isomers have a greater net dipole than trans.

eg. trans-2,3-dichloro-2-butene has no net dipole

Trans isomers have higher boiling point because the molecules can pack better into a crystal lattice due to symmetry.