Ethane methyl groups

Example Hthanc is stable in the staggered gauche) con formation. The transition state for rotating a methyl group in ethane has the eclipsed con form at ion, A gcom etry optim i/ation start in g from an eclipsed eon formation yields th e tran sition state. 

Schematic illustration of the arrangements of ethane molecules in slits of varying sizes. In the slit of width ochJ tich methyl group is able to occupy a potential minimum from the pore (middle). 

A very important characteristic of spin-spin splitting is that protons that have the same chemical shift do not split each other s signal Ethane for example shows only a single sharp peak m its NMR spectrum Even though there is a vicinal relationship between the protons of one methyl group and those of the other they do not split each other s signal because they are equivalent... 

Figure 4.2. Rotational-energy barriers as a function of substitution. Tbe small barrier ( 2kcal) in ethane (a) is lowered even further ( O.Skcal) if three bonds are tied back by replacing three hydrogen atoms of a methyl group by a triple-bonded carbon, as in methylacetylene (b). The barrier is raised 4.2 kcal) when methyl groups replace the smaller hydrogen atoms, as in neopentane (c). Dipole forces raise the barrier further ( 15 kcal) in methylsuccinic acid (d) (cf. Figure 4.3). Steric hindrance is responsible for the high barrier (> 15 kcal) in the diphenyl derivative (e). (After...

Changing the atom bound to a methyl group from carbon to nitrogen to oxygen, as in going from ethane to methylamine to methanol, produces a decrease in the rotational barrier from 2.88 to 1.98 to 1.07kcal/mol. This closely approximates the 3 2 1 ratio of the number of H—H eclipsing interactions in these three molecules. 

The name ethyl is derived from the name of the parent hydrocarbon, ethane. In the same way the name of the methyl group (CH3—) is derived from that of methane, CH4 the name of the propyl group (CH3CH2CH2—) is derived from propane, CH3CH2CH3 etc. 

In ethane, for example, the two halves of the molecule rotate around the bond so that the molecule passes through eclipsed and staggered conformations and all the intermediate orientations. In methanol, the hydroxyl group swings around the methyl group. 

Many molecules are composed of functional groups (hat can rotate with respect to the rest of the molecule. The classical example is ethane, as the possibility of rotation of one methyl group against the other was recognized long ego. Because the torsional mode does not result in infrared activity, its frequency was estimated from thermodynamic data. 

The inherent chemistry above is not altered by substituting the phenyl rings of the ketone or pinacol with methyl groups. The ketone 4,4 -dimethylbenzophenone, as well as the pinacol 1,2,2-tetrakis-(4-methylphenyl)ethane-l,2-diol (TBP) react with U giving tetrakis-(4-methylphenyl)ethylene and l,l, ,2-tetrakis(4-methylphenyl)ethane. A lsl mixture of benzophenone and 4,4-dimethylbenzophenone gives the six expected coupling products. 

An aromatic ring and a double or triple bond in the a-position relative to the C—H bond weaken this bond by virtue of the delocalization of the unpaired electron in its interaction with the iT-bond. The weakening of the C—H bond is very considerable for example, D(C—H) is 422 kJ mol-1 in ethane [27], 368 kJ mol-1 in the methyl group of propene [27] (AD = 54 kJ mol-1), and 375 kJ mol-1 in the methyl group of toluene [27] (AD = 47 kJ mol-1). Such decrease in the strength of the C—H bond diminishes the enthalpy of the radical abstraction reaction and, hence, its activation energy. This effect is illustrated below for the reactions of the ethylperoxyl radical with hydrocarbons ... 

As described in Section 3.4.2, hyperconjugative donor-acceptor stabilizations favor conformers in which one of the rotor C—H bonds eclipses an adjacent double bond. (This is equivalent to an ethane-like staggered preference if the double bond is pictured in terms of two bent banana bonds. ) Hence, in the case of a perfectly localized Lewis structure I, the methyl group would be expected to adopt the preferred pseudo-cA conformation la (with in-plane C—H syn to A=C),... 

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