Pentane intermolecular interactions

Pentane is a nonpolar molecule and is unlikely to have strong intermolecular interactions with polar water molecules. 1-Butanol, however, has an -OH group just as water does, and is therefore a polar molecule that can form hydrogen bonds with water. 1-Butanol is more soluble in water. 

At room temperature, the first four members of the alkane family are gases. The straight chain alkanes from pentane and up are liquids, and octadecane (18 carbons) and up in the alkane family are solids. As the number of carbons increases, the boiling point increases. Branched alkanes have lesser boiling points than their unbranched or less branched isomeric counterparts. The reason for this is that the unbranched molecules have more intermolecular interactions than the branched ones. 

To look into this further, we show in Fig 4, in part (a), the behavior of the heat capacity of polypropylene, in units of J/K.(mol of -CH2-CH2(CH3)- repeat units) (35,36) in comparison with that of the molecular liquid 3-methyl pentane (37) (divided by 2 to have the same mass basis as the polymer repeat unit) (38). It is seen that the liquid heat capacity of the hexane isomer (x 0.S) falls not much above the natural extrapolation to lower temperatures of the heat capacity per repeat unit of the polymer. This implies that the main effect of polymerization, as far as the change in heat capacity at Tg is concerned, is to postpone the glass transition until a much higher vibrational heat capacity has been excited. This not only reduces the value of ACp but has a disproportionate effect on the ratio Cp,i/Cp,g at Tg. This happens despite a lower glassy heat capacity in the polymer than in the molecular liquid at the same temperature. The latter effect is a direct consequence of the lower Debye temperature (and lower vibrational anharmonicity) at a given temperature for in-chain interactions in the polymer than for intermolecular interactions in the same mass of molecules. 

In Fig. 2, the relative permittivities (static dielectric numbers) e of carbon dioxide [23], argon [24], and liquid pentane [25] are plotted against pressure p up to 200 MPa. Even at the highest pressures corresponding to liquid-like densities, e (CO2) is smaller than 1.8, and thus nearly equal to that of a liquid alkane (such as pentane). Since CO2 molecules do not have any permanent electrical dipole moment, the polarization is more or less restricted to the contributions of the electrons and the nuclei. Therefore, typical solvation effects are normally less important, and the intermolecular interactions are predominantly of van-der-Waals type with some higher electrostatic such as quadrupolar interactions. 

The more extended molecule pentane has a much greater surface area and has greater intermolecular interactions (bp 36.1 °C mp-130 °C)... 

Ans. Propanoic acid > 1-butanol = butanal pentane. Pentane has negligible water solubility since there is negligible intermolecular attraction between the nonpolar alkane and the highly polar water. Butanal and 1-butanol have significant solubilities in water since each can hydrogen-bond with water. Propanoic acid is somewhat more soluble in water than either the aldehyde or alcohol since it has two sites (carbonyl and hydroxyl) for interaction with water. 

Ans. Butanoic acid > 1-pentanol > pentanal > methyl propanoate > hexane. The order is based on differences in intermolecular forces since the compounds have very nearly the same molecular mass. Butanoic acid and 1-pentanol have hydrogen bonding but that in the acid is more extensive because of the special interaction between the carbonyl and HO groups. Aldehydes boil higher than esters because the polarity of the carbonyl group is greater than that of an ester group. The attractive forces in alkanes, the dispersion forces, are the weakest. 

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