Question

1. How can you separate nonpolar amines from other organic compounds during an extraction?

2. What is the purpose of exhaustive methylation in elimination reactions?

3. Why do alcohols have higher boiling points than amines?

Answer 1

1.When separating mixtures of organic compounds that have acidic or basic functional groups, such as carboxylic acids, phenols (acidic) and amines (basic), one can exploit the different solubility properties of their protonated and non-protonated forms. For instance, an organic acid is often insoluble in water but soluble in a less polar organic solvent, such as ether. In a separatory funnel containing ether and water, it would reside in the ether layer. When a basic solution is added to the funnel, the acidic compound is deprotonated and becomes an ionic salt. It is now soluble in water but insoluble in ether. At this point, the de-protonated organic salt would reside in the aqueous layer. A similar “solubility switch” exists for basic organic compounds. A water insoluble organic base can be protonated by reaction with an acidic solution and can thus move from the ether layer into the aqueous layer in a separatory funnel. Many of the carboxylic acids are strong enough that they can be deprotonated by a saturated solution of sodium bicarbonate, a relatively weak base. Phenols, on the other hand, require a stronger basic solution, such as aqueous sodium hydroxide, to be deprotonated. Hydrochloric acid is generally used to protonate amines. The solid forms of the acidic and basic organic compounds can be recovered from the aqueous solution using the same solubility switch principles. When acid is added to an aqueous solution that contains the salt of a deprotonated organic acid, the organic acid is re-protonated. In this form, it is now water insoluble and precipitates from the aqueous solution as a solid, which can be collected by vacuum filtration. Similarly, the salt of a protonated amine can be deprotonated by addition of base, at which point it will precipitate from the aqueous solution and can be collected in its solid form by filtration. In some cases, a neutral organic compound, one which has neither an acidic nor a basic functional group, is present in the mixture. Such a compound would remain soluble in the non-polar organic solvent throughout any extractions with acid and base solutions. At the end of a procedure to separate the components of a mixture, the neutral compound can be recovered in its solid form by evaporation of the organic solvent.

Dissolve mixture in ether RCOOH RH carboxvlic acid phenol "neutral" amine Extract with HCl Organic layer Aqueous layer RH Extract wth NaHCO Add NaOH Organic layer Aqueous layer collect precipitate ArOH Extract with NaOH Add HC OOH Organic layer Aqueous layer collect precipitate evaporate solvent Add HCl collect precipitate

2.

Hofmann elimination, also known as exhaustive methylation, is a process where a quaternary ammonium reacts to create a tertiary amine and an alkene by treatment with excess methyl iodide followed by treatment with silver oxide, water, and heat.

After the first step, a quaternary ammonium iodide salt is created. After replacement of iodine by a hydroxyl anion, an elimination reaction takes place to form the alkene.

With asymmetrical amines, the major alkene product is the least substituted and generally the least stable, an observation known as the Hofmann rule. This is in direct contrast to normal elimination reactions where the more substituted, stable product is dominant (Zaitsev's rule).

3.

trans:cis = 60:40 N ー \ Mel, MeOH Ago heat, vacuum AgNO, H2O NH OH

Amines generally have lower boiling points than alcohols of comparable molar mass because amines have weaker hydrogen bonds than alcohols.

Consider the compounds methanol and methylamine.

Methanol, CH3OH: molar mass = 32 g/mol; boiling point = 65 °C

Methylamine, CH3NH2: molar mass = 31 g/mol; boiling point = -6 °C

Methanol has strong hydrogen bonds.

The strong intermolecular forces give methanol a high boiling point.

It is a liquid at room temperature.