Boiling points?

Question

Ethylene glycol (HOCH2CH2OH), the major substance in anitfreeze, has normal boiling point of 198 C. By comparison, ethyl alcohol (CH3CH2OH) boils @ 78 degree C at atmospheric pressure. ethelyne glycol dimethyl ether (CH3OCH2CH2OCH3) has a normal boiling point of 83C & ethyl methyl ether (CH3CH2OCH3) has a normal b.p of 11C.

Question: 1) explain why replacement of a hydrogen on the oxygen by CH3 generally results in a lower b.p.


2) What are the major factors responsible for the difference in b.p . of the two ethers?

Please help lost.

Answer 1

1. This is due to the hydrogen bonding that is very strong in ethylene glycol compared to ethanol. Hydrogen bonding occurs when there is a strong electronegative atom attached to the hydrogen at the end of the molecule. But in this case both the molecules have it but since ethylene glycol has -OH group attached to it at both the ends so it has stronger hydrogen bonding and with higher the hydrogen bonding greater the BP.
2. Iam not sure about the first ether name and the formula, But if it is somewhat similar to it then the answer goes like this. The bulkier the group attached to the O in the ether the greater the Bp.

Answer 2

Water has a higher freezing point and a melting point because of the very strong attraction between water molecules, giving rise to the formation of hydrogen bonds. But it's surprising to find that RMM (relative molecular mass) of similar other compounds like water, which has H2O=18 compared with Methane (RMM=16), Ammonia (RMM=17) are ALL gases at zero degrees except water, which is at it's solid state. The reason is as above, in addition, it has a high latent heat of vaporization, requiring a GREAT deal of energy to over some the forces of attraction between the molecules, allowing them to change from solid to liquid, and liquid to gas. Hope this helps!

Answer 3

replacement of a hydrogen on the oxygen by CH3 generally results in a lower b.p. because you remove the possibility of the molecules to create hydrogen bonds. Interactions between the molecules of the ethers are only of dipole-dipole (Van der Walls interactions), and this kind of bonds are less energetic than hydrogen bonds, so the b.p. is lower because it needs less energy to change state (from liquid to gas).

The 2 ethers are: first 2 different Molecular weight, higher is the MW, higher the b.p.

second the DME ether is symmetrical and ME ether is unsymmetrical, symmetrical molecules normally have better propagation of dipole-dipole bonds (better order) and so this can change the boiling point, even if the MW is the same (think about isomers for example)
So there is a relation between the dipole moment of a molecule and its b.p. and the m.p. too

Answer 4

Question 1 is easier. OH groups can hydrogen bond with each other, and this is relatively hard to break by heating, so BP is higher. Replacement to OCH3 groups severely reduces hydrogen bonding potential.

Question 2: Length is the main reason. Ethers behave essentially like hydrocarbons of the same length. There is no hydrogen bonding. The trend of higher BP with length in alkanes is also seen in ethers. The primary intermolecular forces here are van Der Waals and London dispersion forces.

Answer 5

1. In glycol, you have molecules hydrogen bonded to one another. The boiling point is high, because it takes a lot of energy to get them apart. With glycol methyl ether, you lose that hydrogen bonding.

2. The methyl ethyl ether has a much lower molecular weight.