The difference in electronegativity between hydrogen and carbon is not great and so we do not expect to have significant polarity in the molecule. The only significant intermolecular forces would be the van der waal's and the London's intermolecular forces. These are short distance forces and their magnitude is directly proportional to the surface area. Their forces of attraction between the molecules would increase with increasing size of the molecule; that is with increasing number of carbon.

SIZE AND BOILING POINTS

Tutorial 1.3

Plot the variation of the physical properties against the number of carbon of the alkane and discuss the observation. Would you expect 2-methylpropane to have a higher or lower boiling point than butane, both being C4 alkanes? Hint
Answer

The C1 to C4 alkanes are gases at SATP. Above this, till about 10 units of carbon, they are liquids. The larger alkanes are oils, then grease, and finally wax (candles). Polyalkanes (polyethylene and polypropylene) with more than about 200 units of carbon are plastics used to make supermarket bags, non-transparent bottles, plastic tables, chairs, buckets, road barricades, doll-houses, etc. They are the most important commercial plastics in terms of quantity produced. Please dispose off your plastics in the proper manner. They might not be toxic but animals have been known to swallow them and suffered for it. They are also the most common cause for floods in cities because they clot up the drainage system.

Plastics can actually be recycled. One easy way is to reuse them. If everyone would use half the number of plastic bags every time they visit the supermarket, we can make our natural resource last twice as long.

The physical properties of the alkenes and alkynes paralleled that of the alkanes; reflecting the dependence on the van der waal's and the London's intermolecular forces. The lower members are gases.

The smaller alkanes are soluble in non-polar solvents, and the polyalkanes are unreactive to acid, base, polar and non-polar solvents. This is the reason for the vast application of polyalkanes. (Which is best for storing sodium hydroxide in the laboratory; glass bottle or polypropylene bottle?)

POLARITY

Alkyl Halides

An easy way (also a cheap way) to introduce a polar group in hydrocarbon is by chlorination. Chlorine can be obtained from the electrolysis of sea water.

Chlorine is much more electronegative than carbon, so the electrons in the bond will tend to spend more time with the chlorine atom, resulting in an unequal sharing of the electrons of the bond. We represent the polar nature of the bond by;

1 Debye (D) dipole moment is defined as two opposite charges of 10‾¹⁰esu magnitude separated by a distance of 10‾⁸cm. The dipole moments for some carbon-halogen bonds are:

Carbon-halogen	Bond length (A)	Dipole moment (D)
CH3−F	1.39	1.82
CH3−Cl	1.78	1.94
CH3−Br	1.93	1.79
CH3−I	2.14	1.64

Although the electronegativity decreases from fluorine to iodine, the large difference in bond distance between fluoride and chloride resulted in the chloride being the more polar. The dipole moment than decreases on both side of the chloride.

The physical attraction between the halide molecules can be seen from their boiling points.

Boiling Point (�C)

The increase in boiling point for the fluorides and chlorides as compared with the alkanes can be easily explained by the dipole moment present in the halides.

Since the dipole moments for the bromides and iodides are less than the chlorides the attractive forces for these halides should decrease, not increase. So now we have to consider another property of halogens. With increasing size of the halogen the attraction between the protons in the nucleus and the valence electrons is reduced. So the valence electrons, especially lone-pair electrons, tend to be attracted by positive centres in the system. This is known as polarizability of valence orbitals. The momentary positive centre in the carbon is greatly magnified towards the valence electrons of the bromide and iodine, resulting in great attraction between them. This effect has been shown by bromide and iodide in organic chemistry.

Aldehydes and Ketones

The polarity for a carbonyl group is even greater than for the alkyl halides. This is because a π−bond is more polarizable than a σ−bond. The dipole moment for formaldehyde is 2.27D and for acetone it is 2.85D. Taking account of the molecular weight it is reasonable to expect that the boiling point for aldehyde and ketone be about that of the corresponding alkyl chloride.

Boiling Point (�C)

Alkyl chloride			Aldehyde			Ketone
CH3Cl	− 24.2		HCHO	− 21.0
CH3CH2Cl	12.3		CH3CHO	21.0
CH3CH2CH2Cl	46.6		CH3CH2CHO	49.0		CH3COCH3	56.0
CH3CH2CH2CH2Cl	78.4		CH3CH2CH2CHO	76.0		CH3CH2COCH3	80.0

The primary force for hydrocarbon intermolecular attraction is the van der waals forces. So the boiling point of a hydrocarbon increases with increase in size. Dipolar attractions are of a greater magnitude than the van der waals forces and so there is a large increase in the boiling point for comparable hydrocarbons.

But the magnitude of attraction between molecules that form hydrogen bonding is even greater . This intermolecular interaction is so "strong" that we even "mislabelled" it as a bond. Actually it is just an electrostatic attraction with a value of about 21 kJ mole‾� as compared to about 410 kJ mole‾� for σ−bond. So the boiling points of compounds having hydrogen bonding - alcohols and amines - are greatly increased as compared to a hydrocarbon of similar size.

Since the nitrogen is less electronegative than oxygen (remember electronegativity increases from left to right across the Periodic Table), the hydrogen bonding for amine is not as strong as for alcohol and so for an equivalent chain length the boiling point is lower for amine.

Alcohols and amines can also form hydrogen bond with water, so they are miscible. However as the size of the hydrocarbon component increases the influence of the OH group is finally unable to determine its solubility. So the solubility of n-butanol in water is only about 1.1 mole dm‾� and decreases to 6.9 x 10‾� mole dm‾� for n-hexanol.