Haloalkanes & Their Boiling/Melting Points

How does the halide on a haloalkane affect its melting point?

& Is it right to say that bigger haloalkanes have more Van Der Waals hence have greater melting and boiling points because more heat energy is required etc etc

Reply 1

ayesha2210

the bigger the halide, the higher the melting point
bigger halides have stronger permanent dipoles and so yes the attractive forces between these molecules would be more :smile:

Reply 2

Kyalimers

ayesha2210

the bigger the halide, the higher the melting point
bigger halides have stronger permanent dipoles and so yes the attractive forces between these molecules would be more :smile:

Umhmmmmmm

Reply 3

Masamune88

ayesha2210

the bigger the halide, the higher the melting point
bigger halides have stronger permanent dipoles and so yes the attractive forces between these molecules would be more :smile:

Ok so the bigger the haloalkane the higher the melting point.

I don't get it with the halide ion there though - do the bigger halide ions not have lower electronegativities and hence would have weaker permanent dipole interactions between molecules?

And if your talking about longer chains would that not weaken permanent dipole interactions because of an increased distance between them - do we not use increased no. of Van Der Waals forces as the main factor?

I would have though it was smaller ions ie fluorine and chlorine would have hgher melting points because of the increased electronegativity and hence stronger permant dipole interactions - more heat energy is required etc. etc?

Reply 4

Kyalimers

OKOK - You neg repped me for saying umhmmmmmmm so i shall help you.

The larger a molecule is, the bigger the van der Waals force it has. That is just a general rule of thumb. Now, we know that Halides occur diatomically i.e.

F_2 or Br_2

and going down the group the molecules get bigger: therefore there is more van der Waals.

Chain length never really changes the melting or boiling ponts of anything. It is the type of bond and the position of the bond that matters: now I hope you are happy!!!

EDIT: be wary. You are talking about HALIDES not halogens!!!

Reply 5

ayesha2210

I don't get it with the halide ion there though - do the bigger halide ions not have lower electronegativities and hence would have weaker permanent dipole interactions between molecules? -

I understand that the smaller halogens are the more electronegative ones, BUT when it comes to this question the boiling points increase with bigger halogens as the number of electrons in the molecule increases.

And if your talking about longer chains would that not weaken permanent dipole interactions because of an increased distance between them - do we not use increased no. of Van Der Waals forces as the main factor? -

wel wen they talk about longer chains they mean more repeated sequences and so more halogen atoms are present - and so more dipole interactions and so more energy required to separate etc etc

hope this helps man!

Reply 6

charco

Study Forum Helper

Halide IONS cannot have any electronegativity as the concept of electronegativity refers to the ability to attract electrons within covalent bonding.

There are two fundamental types of intermolecular force:
1. Van der Waals forces - all covalent molecules
2. Permanent dipole - dipole interaction (force) - only polar molecules

Halogens (elements) are diatomic molecules and have only Van der Waals forces. These are proportional to the relative mass of the molecules and increase down the group.

Hydrogen halides are polar, they have both Van der Waals forces AND permanent dipole-dipole forces. The Van der Waals force increases down the group BUT the permanent dipole-dipole force decreases. The permanent dipole forces are called hydrogen bonding when fluorine is attached to hydrogen atoms, as fluorine is highly electronegative. Hydrogen bonding is a special case of dipole -dipole force that only occurs when hydrogen atoms are covalently bonded to either F, O or N atoms.

In the case of HF the permanent dipole -dipole force is very strong and more than compensates for the weak Van der Waals force. HF has a higher b.p. than HCl. HBr has weaker dipole -dipole than HCl BUT much larger Van der Waals. HBr has a higher b.p. than HCl

Reply 7

Gaurav07071981

What to say about the increase or decrease of haloalkane's BP ...if it's both chain is decreasing and the halogens size is increasing ???

Reply 8

shengoc

Original post by Gaurav07071981

What to say about the increase or decrease of haloalkane's BP ...if it's both chain is decreasing and the halogens size is increasing ???

You are talking of two CHANGES. and their effects on the van der Waals forces are contradictory, depending on what you are specifying exactly.

when you said "chain is decreasing" - are you talking of isomeric (ie still same number of atoms throughout but branched) or are you talking of homologous series, i.e. actually overall molecule is shorter?