Question

The following shows some organic compounds. CH3CH2OCH2CH3,CH3CH2CH2CH2OCH, CH3CH2CH2CHO,CH3CH2CH2CH2CH3, C(CH3)4, CH3CH2CH2CH2Cl, (CH2)4O a.Which of the listed molecules has the highest boiling point? Explain your answer with molecular structure. b. Which of the listed molecule has the highest vapor pressure at 25C? Explain your answer with molecular structure. c. Diethyl ether (CH3CH2OCH2CH3) and butyraldehyde (CH3CH2CH2CHO) are both polar compounds, but diethyl ether has much lower boiling point than butytaldehyde. Explain why with molecular structures.

Answer 1

(a)

Boiling point of an organic compound raises if the following forces are present between its molecules:

(i) Dipole dipole forces: It is present if the molecule is polar. If polarity is higher, the dipole forces are also higher.

(ii) London dispersion forces: It is present in all molecules, and it increases with the increase in the molecular mass.

(iii) Hydrogen bond: It is present if the molecule contains a hydrogen atom attached directly to F,O or N. It is stronger than the above two intermoleculae forces.

Among the compounds given to us:

CH3CH2OCH2CH3 is an ether.

CH3CH2CH2CH2OCH is an aldehyde.

CH3CH2CH2CHO is also an aldehyde, but with smaller molecular mass as compared to the one above.

CH3CH2CH2CH2CH3 is an alkane.

C(CH3)4 is a an alkane, called neopentane.

(CH2)4O is a cyclic ether.

Since aldehydes are more polar as compared to ethers, they have higher boiling point.

Alkanes are non-polar, so they also have lower boiling points.

Thus alkanes and ethers are out of the list.

Alkyl halides are less polar than aldehydes, but they do have a polarizable H-present in them, so they have slight hydrogen bonding.

So the boiling points of alkyl halides and aldehydes of comparable masses are almost similar.

But if we compare an aldehyde with higher MW to an alkyl halide with smaller MW, then the aldehyde has higher boiling point.

So keeping all these factors in mind, the compound with highest boiling point is CH3CH2CH2CH2OCH, also called as pentaldehyde.

(b)

And among these compounds, the compound with absolutely no polarity will have lowest boiling point.

This means we have two possible candidates for lowest boiling point: CH3CH2CH2CH2CH3 and C(CH3)4.

Since CH3CH2CH2CH2CH3 has no branching, it has more surface area available for interaction and hence shows higher intermolecular London dispersion forces.

Thus, the compound with smallest boiling point is C(CH3)4.

The smaller the boiling point, it means lesser energy is required to pull the molecules apart from liquid into the gaseous state, so at a given temperature more molecules are present in gas phase for a compound with small boiling point. This means it also has highest vapor pressure.

So the compound with highest vapor pressure at 25⁰C is C(CH3)4.

(c)

This is because aldehyde has more polarity as compared to an ether.

In an ether, the two alkyl groups attached to the oxygen atom are aligned in opposite direction to each other. So they tend to cancel the polarity of each other, so the net effect is that the total polarity of the ether decreases.

While in aldehyde, there is no such case. The C=O double bond is highly polar.

More polarity means that intermolecular forces are stronger, so the boiling point is also higher.

Thus diethyl ether has lower boiling point than butyraldehyde although they both have similar molecular masses.