Bent hydrocarbons

Bent hydrocarbon chains - e.g. brassidic acid trans-CH3(CH2)7CH=CH(CH2)iiCOOH), which has a straight hydrocarbon chain, gives a condensed film, whereas erucic acid (cis-CH3(CH2)7CH-CH(CH2)iiCOOH), which has a bent hydrocarbon chain, gives a very expanded Aim. 

The formation of such films are favored by (1) bulky head groups, (2) more than one polar group, (3) more than one hydrocarbon chain orientated in different directions from the polar part of the molecule (e.g., esters), (4) bent hydrocarbon chains such as ds-unsaturated acids, and (5) branched hydrocarbon chains. 

Thermolysis of m-PtH(CH2CMe3)(cy2PC2H2Pcy2) at 45-80°C yields a bent platinum(0) complex (Figure 3.13) that is intensely reactive to a whole range of unactivated C—H bonds in saturated and unsaturated hydrocarbons. 

Figure 16. Time response of a bent PCS fibre to liquid hydrocarbons and petrol with and without the output mode filter.

Calculations have shown that there are two ways in which cyclopropane may be protonated at one of the carbons (comer protonated) and at one of the C—C bonds (edge protonated). They have about the same energy, and are involved in the movement of a proton around the ring. The calculations indicate that cyclopropane is unusually basic for a hydrocarbon. The reason can be seen in the stmctures of the protonated cyclopropanes (Fig. 15.4). The bent bonds permit a proton to bond to a C C bond without coming close to the positively charged carbon nuclei. 

Distortion is less in the [3.3] paracyclophane, where the inter-ring separation varies between 3.1 and 3.3 A, and the benzene rings are only bent by 6°. In the crystal, the hydrocarbon links adopt the chair conformation, and the benzene rings are displaced by about 0.5 A from the sandwich structure, although the rings remain parallel. Some ground-state overlap occurs between the rings, since the UV absorbance extends to the red of 305 nm. The peak of the broad fluorescence emission of [3.3] paracyclophane appears at the same position as that of [2.2] paracyclophane. 

A useful comparison may be made between bent, flat, and linear molecules by considering the diffusion coefficients for ethane, ethene, and ethyne. In the sinusoidal channel, ethane diffuses the slowest, ethene approximately 30% faster, and ethyne 3 times as fast. In the straight channel and parallel to the z-axis, ethene and ethyne both diffuse approximately 3 times faster than ethane. These ratios are consistent with the relative cross-sectional area of the three C2 hydrocarbons. 

Fig. 1-4. Ball-and-stick model of hydrocarbons with bent bonds.

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