Chemistry (P1/2) - Acidic and Basic Characteristics of Organic Molecules

conjugative effect

instance in which the pi orbitals of multiple atoms in a molecule overlap and a delocalized system of electrons is formed between them

Describe the conjugative effect as seen in a carboxylate ion

In a carboxyl group, each oxygen atoms overlaps with the carbon atom — the sigma and pi bond from the carbonyl oxygen and the sigma bond from the hydroxyl oxygen. These overlaps create a larger electron cloud for electrons to spread their density, making the carboxylate ion more stable than the carboxyl group

inducive effect

instance in which the difference in polarity between two species in a bond causes electrons to tend to a particular species, increasing its electron density

negative inducive effect

instance in which an electron-rich species is bonded to an electron-deficient species and loses electrons

positive inducive effect

instance in which an electron-deficient species bonds with an electron-rich species and gains electrons

Explain how the conjugative effect affects the acidity of carboxyl acids

The pi orbitals of the oxygen atoms each overlapping with the pi orbital of the carbon atom creates a large region over which the negative charge of the molecule can be distributed more widely. Since this dynamic, as a carboxylate ion, is more stable than the carboxyl group, this is a means by which carboxylic acids can act as proton donators as the hydrogen atom would be ejected. However, carboxylic acids are still relatively weak acids because the complete disassociation of the carboxyl group into the carboxylate and hydrogen ions requires energy due to the strength of the O-H bond resulting in an equilibrium in which the O-H bond is constantly breaking and reforming, making it so that only a fraction of carboxyl groups are deprotonated at given point.

Explain how the inducive effect affects the acidity of carboxylic acids.

The electron-deficiency of the alkyl carbon the carboxyl group is bonded to determines its ability to protonate. The greater the electron-deficiency of the alkyl group, the more the carboxyl group shifts its electron density towards it which in turn would make the carboxylate ion more stable as the negative charge would be better distributed and make deprotonation more frequent. Conversely, the lesser the electron-deficiency of the alkyl group, the more the electron density of the alkyl group shifts towards it which in turn would make the carboxylate ion less stable as the negative charge would be distributed less favourably.

Explain how the conjugative effect affects the acidity of phenols

When phenols are deprotonated, the remaining phenoxide ion is stabilized because the negative charge resonates through the ring, distributing the electron density across the ion. However, the negative charge is distributed between the oxygen atom and the carbon atoms in the ion, which is unfavourable as the aromatic p orbitals of the carbon atom are at a higher energy than the lone pair p orbitals in the oxygen, making the resonance effect less effective than that in the carboxylate ion, where negative charge is evenly distributed, which in turn centers high electron density around the oxygen making re-protonation likely

Explain how the inducive effect affects the acidity of phenols

The inducive effect can increase the acidity of phenols in the instance in which an electron-deficient group can pull electron density away from the ring which would reduce electron-electron repulsion with the oxygen’s lone pair, reduce the likeliness of re-protonation and better distribute the negative charge in the phenoxide ion.

Explain how the conjugative effect affects the acidity of alcohols

A primary reason why alcohols do not display acidic character is the fact that there is often no system of pi orbitals to delocalize negative charge and stabilize the conjugate base after deprotonation and the negative charge is typically centered entirely on the oxygen atom which would make re-protonation extremely likely

Explain how the inducive effect affects the acidity of alcohols

The acidity of alcohols could be increased if an electron-withdrawing group was present to draw electron density from the O-H bond and consequently making deprotonation easier.

aliphatic compounds

those that form open chains.

Explain how aliphatic amines are able to display basic character

The presence of a lone pair on the nitrogen atom can accept a hydrogen atom.

Explain why primary and secondary aliphatic amines are stronger bases than ammonia

Methyl groups push their electron density towards the nitrogen atom — an example of the positive inducive effect — allowing it to more readily attract protons than the nitrogen atom in ammonia

Explain the basic strengths of primary, secondary and tertiary aliphatic amines in relation to each other

The stability of the conjugate acids is determined by the ability of water molecules to solvate the ion and stabilize its positive charge effectively. The presence of two methyl groups makes it physically more difficult to solvate the conjugate bases formed from secondary aliphatic amines than in primary aliphatic amines as the molecule is more bulky. However, a stronger inducive effect due to the higher amount of electron-donating groups makes their strengths more comparable and typically case-by-case.

However, despite having the most electron-donating groups, tertiary aliphatic amines are typically weaker than their primary and secondary counterparts because its bulkiness makes it more difficult for water molecules to solvate the conjugate acid it forms to stabilize its positive charge effectively and even makes it harder for a proton to approach the nitrogen atom in the first place.

Explain how the conjugative and inducive effects affect the basicity of aromatic amines

The p orbital holding the lone pair electrons of the nitrogen atom overlaps with the ring, increasing the conjugative effect and shifting the electron density away from the nitrogen atom as the conjugative effect is stronger than the inducive effect that would otherwise push electron density towards the nitrogen atom, making it more difficult for protons to be attracted to nitrogen atoms. This makes aliphatic amines weaker than ammonia.

Explain why amides do not display basic character

The p-orbital of the nitrogen atom overlaps with the p-orbital of the carbon in the carbonyl group and electrons are then delocalized between the O, C and N atoms which increases the conjugative effect stabilizing the molecule to an extent that the nitrogen losing its lone pair (for example by protonating) would be energetically infavourable as it would disrupt the resonance. In addition, the carbonyl group inducively draws electrons away from the nitrogen atom, making protonation on the nitrogen atom extremely unlikely.

amino acids

organic compounds that are used to make proteins which contain both the —NH2 and —COOh groups

zwitterion

electrically neutral species that is formed with a positive and negative ion in two different parts of the same species.

Describe the behaviour of amino acids in neutral, acidic and alkali solutions

In neutral solutions, both group ionize and the solution is electrically neutral

In acidic solutions, both groups ionize but the negative —COO group accepts a proton, making the solution electrically positive.

In alkai solutions, both groups ionize but the positive —NH3 group donates a proton to the hydroxide ion, making the solution electrically negative