Useful Concepts:

The following is a summary of the physical properties of the differing functional groups.

Alkanes:

The low polarity of all the bonds in alkanes means that the only intermolecular forces between molecules of alkanes are the very weak induced dipole - induced dipole forces (London forces).  These forces are easily overcome.  As a result, in comparison with other functional groups, alkanes tend to have low melting and boiling points and very low solubility in polar solvents such as water (remember "oil and water don't mix" and the adage "like dissolves like").

Alkyl Halides:

The polar bond creates a molecular dipole that raises the melting points and boiling points compared to alkanes.

Alkenes:

As with hydrocarbons in general, alkenes are non-polar and are insoluble in water but soluble in non-polar organic solvents.

Alkynes:

As with hydrocarbons in general, alkynes are non-polar and are insoluble in water but soluble in non-polar organic solvents.

Alcohols:

• The polar nature of the O-H bond (due to the electronegativity difference of  the atoms) results in the formation of hydrogen bonds with other alcohol molecules or other H-bonding systems (e.g. water). The implications of this are:
• high melting and boiling points compared to analogous alkanes.
• high solubility in aqueous media.

Benzene:

In the absence of polar substituents, arenes are typical of hydrocarbons: low melting and boiling points, low solubility in polar solvents.

Organometallic Compounds:

Organometallic are usually kept in solution in organic solvents due to their very high reactivity (especially with H₂O, O₂ etc.)

Thiols:

• Hydrogen bonding is much weaker than that in alcohols.
• Lower boiling points than similar alcohols.

Ethers:

• The polar nature of the C-O bond (due to the electronegativity difference of the  atoms) results in intermolecular dipole-dipole interactions.
• An ether cannot form hydrogen bonds with other ether molecules since there is no H to be donated (no -OH group).
• Ethers can be involved in H-bonding with systems able to donate H (e.g. water).
• The implications of these effects are:
• lower melting and boiling points compared to analogous alcohols.
• solubility in aqueous media similar to analogous alcohols.

Epoxides:

• Similar to analogous ethers
• Act as Lewis bases, form complexes with metal ions.

Sulfides:

• The polar nature of the C-S bond (due to the electronegativity difference of the  atoms) results in intermolecular dipole-dipole interactions.
• A sulfide cannot form hydrogen bonds with other sulfide molecules since there is no H to be donated (no -SH group).
• Sulfides can be involved in H-bonding with systems able to donate H (e.g. water).
• The implications of these effects are:
• lower melting and boiling points compared to analogous alcohols and thiols.
• solubility in aqueous media similar to analogous alcohols and thiols.

Aldehydes and Ketones:

• The polar nature of the C=O (due to the electronegativity difference of  the atoms) means dipole-dipole interactions will occur.
• Though C=O can not hydrogen-bond to each other, the C=O can accept hydrogen bonds from hydrogen bond donors (e.g. water, alcohols).

The implications of these effects are:

• higher melting and boiling points compared to analogous alkanes.
• lower boiling points than analogous alcohols.
• more soluble than alkanes but less soluble than alcohols in aqueous media.

Carboxylic Acids:

• The polar nature of both the O-H and C=O bonds (due to the electronegativity difference of  the atoms) results in the formation of strong hydrogen bonds with other carboxylic acid molecules or other H-bonding systems (e.g. water). The implications are:
• higher melting and boiling points compared to analogous alcohols.
• high solubility in aqueous media.
• hydrogen bonded dimers in gas phase and dimers or aggregates in pure liquid.

Acidity:

• Carboxylic acids are the most acidic simple organic compounds (pK_a ~ 5).
• But they are only weak acids compared to acids like HCl or H₂SO₄. (Remember the lower the pK_a, the stronger the acid).
• Resonance stabilization of the carboxylate ion allows the negative charge to be delocalized between the two electronegative oxygen atoms (compare with alcohols, pK_a ~ 16).
• Adjacent electron withdrawing substituents increase the acidity by further stabilizing the carboxylate.

Carboxylic Acid	Structure	pKa
Ethanoic acid	CH3CO2H	4.7
Propanoic acid	CH3CH2CO2H	4.9
Fluoroethanoic acid	CH2FCO2H	2.6
Chloroethanoic acid	CH2ClCO2H	2.9
Dichloroethanoic acid	CHCl2CO2H	1.3
Trichloroethanoic acid	CCl3CO2H	0.9
Nitroethanoic acid	O2NCH2CO2H	1.7

Carboxylic acid derivatives:

• The polar nature of both the C-X and C=O bonds (due to the electronegativity difference of  the atoms) results in the formation of strong hydrogen bonds with other carboxylic acid molecules or other H-bonding systems (e.g. water). The implications are:
• higher melting and boiling points compared to analogous alcohols.
• high solubility in aqueous media.
• hydrogen bonded dimers in gas phase and dimers or aggregates in pure liquid.
• nitriles are weakly polar due to the C≡N bond. The implications are:
• low melting points, comparable to alkanes.
• low solubility in water

Amines:

• The polar nature of the N-H bond (due to the electronegativity difference of  the two atoms) results in the formation of hydrogen bonds with other amine molecules, see below, or other H-bonding systems (e.g. water). The implications of this are:
• high melting and boiling points compared to analogous alkanes.
• high solubility in aqueous media.
 

intermolecular H-bonding in amines

Basicity:

• Amines are more basic than analogous alcohols (R-NH₃⁺  pK_a ~ 10, R-OH₂⁺  pK_a ~ -3)
• Factors (i.e. resonance, electronegativity) that affect the availability of the lone pair will affect the basicity.
• N is less electronegative than O and therefore N is a better electron donor.
• alkyl and non-aromatic heterocyclic amines are slightly stronger bases than ammonia.
• aryl amines are much weaker bases than ammonia, a result of the delocalization of the lone pair into the π system of the ring.


• The anion derived by the deprotonation of an amine is the amide ion, NH₂^-
• Amide ions are important bases in organic chemistry (example).
• Amines react with Na (or K) to give the amide ion.
• The basicity of aryl amines is:

Increased by the presence of electron-donating substituents on the ring, by counter-acting the delocalization of the lone pair into the π system of the ring. Decreased by the presence of electron-withdrawing substituents which enhance the delocalization of the lone pair into the π system of the ring (especially those ortho or para to the amine functional group, see right).

 
• Inclusion of a heteroatom into an aromatic ring generally decreases basicity, unless protonation leads to an ion that can be stabilized by electron delocalization:

Aryl Halides:

• Aryl halides have similar properties to alkyl halides.
• The polar bond creates a molecular dipole that raises the melting points and boiling points compared to similar hydrocarbons.
• Aryl halides tend to be less polar than alkyl halides (since an sp² C is more electronegative than an sp³ C).

• Insoluble in water (low polarity, no hydrogen bonding).
• More dense than water.

Phenols:

• The polar nature of the O-H bond (due to the electronegativity difference of the atoms) results in the formation of hydrogen bonds with other phenol molecules or other H-bonding systems (e.g. water). The implications of this are:

• high melting and boiling points compared to analogous arenes.
• high solubility in aqueous media.


• The presence of intramolecular hydrogen bonding is believed responsible for the significantly lower boiling points of certain ortho-substituted phenols vs the meta- and para- analogs.

Acidity:

• Phenols are more acidic (pK_a ~ 10) than alcohols (pK_a ~ 16 - 20), but less acidic than carboxylic acids (pK_a ~ 5).
• The negative charge of the phenolate ion is stabilized by resonance due to electron delocalization onto the ring as shown below:

• The acidity difference means that it is possible to separate phenols  from alcohols or carboxylic acids.
• Mixing an ether solution, of either phenol and alcohol or phenol and carboxylic acid, with dilute base (sodium hydroxide and sodium bicarbonate, respectively), results in the stronger acid being converted to its alkali salt, which is then extracted to the aqueous phase and can be separated from the organic phase.
• Nucleophilic substitution reactions of  phenols are generally carried out under basic conditions as the phenolate ion is a better nucleophile.

Substituent Effects on Acidity:

Substituents, particularly those located ortho or para to the -OH group, can dramatically influence the acidity of the phenol due to resonance or inductive effects. Electron withdrawing groups enhance the acidity, electron donating substituents decrease the acidity.  The resonance stabilization of o-nitrophenol is shown below:

Compound	pKa		Compound	pKa
Phenol	10.0		m-Nitrophenol	8.4
o-Methoxyphenol	10.0		p-Methoxyphenol	10.2
o-Methylphenol	10.3		p-Methylphenol	10.3
o-Chlorophenol	8.6		p-Chlorophenol	9.4
o-Nitrophenol	7.2		p-Nitrophenol	7.2