An aldehyde consists of a terminal carbonyl group, O=CH
- Explain the differences in the boiling points between methanal and methanol?
Since, methanol is an aldehyde; it has a terminal carbonyl group, resulting in only dipole-dipole forced holding two molecules together. On the other hand, methanol contains a hydroxyl group (since it’s an alcohol), resulting in hydrogen bonding also holding two molecules together. Since hydrogen bonding is much stronger than dipole-dipole, it would take more energy to separate two molecules of methanol than it does to separate two molecules of methanal. Hence methanol would have a higher boiling point than methanal.
- What conclusion can you make about the boiling point differences between alcohols and aldehydes?
Since alcohols involve hydrogen bonding, as well as dipole-dipole, they are much harder to separate than aldehydes which only form dipole-dipole bonds. Hence, in general, alcohols have higher boiling points than aldehydes
- What is the functional group in a carboxylic acid?
A carboxylic acid consists of a carboxyl group, C(=O)OH
- Compare the boiling points of similar masses carboxylic acids with alcohols and aldehydes.
Carboxylic acids have much higher boiling points than alcohols and aldehydes. This is due to a more extensive association of carboxylic acids through intermolecular hydrogen bonding. The O-H bonds between carboxylic acids are more polar than that of the ones between alcohols (because of the electron withdrawing effect of the carbonyl group in O-H)
- Compare the relative polarities of the OH bond and NH bond.
An OH bond is more polar than a NH bond because oxygen has a higher electronegativity than nitrogen, resulting in a higher electronegativity difference between the OH bond, which means oxygen has a greater pull on the electrons than nitrogen does in their respective bonds. Hence the OH bond is polar.
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Draw the H-bonding possible for Ammonia (NH3)
- Are mono, di and tri substituted amines capable of H-bonding?
Mono substituted amines are possible of hydrogen bonding (between the lone pair of the very electronegative of the nitrogen and the slightly positive hydrogen of another molecule) , as our di substituted amines. However, tri substituted amines are not capable of hydrogen bonding.
- Compare the boiling points between ethane, methylamine and methanol. Explain.
Between ethane and methylamine, methylamine has the higher boiling point despite both having the same number of electrons. This is due the fact that methylamine is able to form a hydrogen bond with another methylamine molecule which ethane is unable to do so. However, between methylamine and methanol, the methanol has a higher boiling point as it contains a hydrogen bond between oxygen and hydrogen which is stronger than the hydrogen bond between nitrogen and hydrogen in methylamine.
- Explain the differences in the boiling point and melting point between methylamine, dimethylamine and trimethylamine.
Since methylamine contains the strongest hydrogen bond, it is has the highest melting and boiling point. Since dimethylamine has lower dipole-dipole attractions in terms of the hydrogen bond, it has a lower boiling point then methylamine but higher than the trimethylamine. Since trimethylamine is unable to hydrogen bond, it has the lowest boiling and melting point.
- Explain the differences in boiling point between propanone and 2-propanol
Much like the aldehyde, a ketone (in this case propanone) is unable to form hydrogen bonds hence the boiling point of 2-propanol (the alcohol) is higher than that of the ketone.
- Compare the boiling points of ketones to similar massed alkanes. Explain the differences
Ketone’s boiling points are considerable higher than alkane, indicating the presence of weak intermolecular dipole-dipole forces. The carbonyl group ("carbon double bond oxygen") is polar since oxygen is more electronegative than carbon and forms a partially charged dipole.
- Compare glycerol (1,2,3-propanetriol) to propanol and ethylene glycol (1,2-ethanediol) to ethanol. Explain the boiling point differences between these compounds.
- What is the functional group of ethers?
An ether contains ether group which consists of an oxygen bonded to two alkyl groups.
- Can ethers H-bond together?
No, ether molecules cannot H-bond within each other.
- Compare the boiling points of methoxymethane to ethanol and methoxyethane to propanol. Explain.
The difference in boiling points of methoxymethane and ethanol is that ethanol has a hydrogen atom attached directly to an oxygen and so hydrogen bonding can occur between ethanol molecules. However, in methoxymethane, there aren’t enough hydrogens for hydrogen bonds to form. Hence the boiling point of ethanol is significantly higher. The same concept can be applied to comparing methoxyethane with propanol.
- Compare the cis to trans- 2-butene isomers and pentane to neopentane. Explain the differences in boiling and melting points.
Between the cis and trans isomers
- Compare the structures of cis-1,2-dichloroethene and trans – 1,2 –dichloroethene. Explain the differences in boiling and melting points
The difference between the two can be explained by the fact that in cis form, there are 2 polar C-Cl dipole moments, giving an overall molecular dipole and so when it bonds with other molecules, there are dipole-dipole forces as well as London dispersion which increases the boiling point. However, in the trans form, the two C-Cl bond moments cancel and the molecule is rendered non-polar and so there are no dipole-dipole forces involved hence a lower boiling point.
