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Chemistry 2131:
Organic Chemistry for the Life Sciences(3)
Physical Properties of Alkanes
1. Physical Properties of Alkanes:
- we have learned so far to name and draw the alkanes and cycloalkanes, we have even discussed their conformations, we have yet to discuss their physical properties. Let's start by looking at the melting point and boiling point data for the common straight chain alkanes
Name mp (C) bp (C) density at 0 C (g/ml) methane -182 -164 gas ethane -183 -88 gas propane -190 -42 gas butane -138 -0 gas pentane -130 36 0.626 hexane -95 69 0.659 heptane -90 98 0.684 octane -57 126 0.703 nonane -51 151 0.718 decane -30 174 0.730 - inorder for a substance to be able to exist as a liquid or a solid there must be intermolecular forces of attraction between particles of the pure compound. All of these forces are electrostatic in nature, but the strength of these interactions vary considerably with the type.
- the strongest attractive forces are those between ions. Weaker interactions include dipole-dipole interactions and hydrogen bonding. We will spend considerable time on these types of interactions later in the course, but for now they are unimportant because we have no dipoles or hydrogen bonding groups (you need hydrogen bonded to an electronegative atom such as nitrogen or oxygen).
- this leaves us with the weakest of attractive forces, called dispersion forces or van der Waal's interactions. These can occur between nonpolar molecules such as hydrocarbons, especially when they are tightly packed together (dispersion forces are very distance sensitive).
- so, what are these mysterious dispersion forces? To approach this you have to think about the instantaneous distributions of electron densities rather than the average distribution (their orbitals for instance). Averaged over time, nonpolar molecules have symmetrical distribution of electrons, that is to say there is no dipole moment. However at any instant in time, there is a probability that the electron density is shifted one way or another to give a brief polarization of charge or dipole. This temporary dipole can induce a similar dipole in an adjacent molecule. So, dispersion forces are weak electrostatic interactions that occur between temporary induced dipoles of adjacent molecules.
- the strength of dispersion forces is related to how easily a given electron cloud can be polarized. Remember how we said that the electrons in small atoms and molecules tend to be held close their nuclei? These electrons are not as easily polarized. Electrons in larger atoms are more easily polarized. So, as molecular size and mass increases, the dispersion forces get stronger.
- another important point is that dispersion forces are very sensitive to distance. They vary with 1/d6. The long and the short of it is that for dispersion forces to be important, the molecules must be very close together, essentially in contact with each other.
- what does this have to do with the physical properties of alkanes? Everything. Alkanes are nonpolar molecules and dispersion forces are the only attractive forces between them. Because these dispersion forces are so weak, the boiling points of alkanes are uncharacteristically low for their molecular weights.
- however as the molecular weight increases, the dispersion forces increase. Thus, the boiling points of alkanes increase with molecular weight.
- the trend for melting points is less consistent because although dispersion forces are essential here too, packing of the molecules into regular arrays must also occur, and this is less easy to predict with size
- note that the density also increases with molecular weight, suggesting that the molecules are held more tightly together
- so far we have only looked at the straight chain alkanes, what aeffect does branching have on these physical properties? This is well demonstrated by looking at a set of constitutional isomers of the formula C6H14.
name mp (C) bp (C) density hexane -95 68.7 0.659 2-methylpentane -154 60.3 0.653 3-methylpentane -118 63.3 0.664 2,3-dimethylbutane -129 58.0 0.661 2,2-dimethylbutane -98 49.7 0.649 - branching reduces the boiling point, the more branching the lower the boiling point. The differences in boiling points are related to molecular shape. As branching increases the shape of the alkane becomes nore compact, less surface area. As the surface area decreases the contact between adjacent molecules decreases, so the dispersion forces decrease, so the boiling point decreases.
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