Acetaldehyde rotational barrier

Relative Rotamer Stabilities and Rotational Barriers for 2-Substitnted Acetaldehydes (kcal/mol)... 

As a consequence of the energy decrease in both u and 7r, only the —rr interaction need be considered when dealing with the rotational barrier in CH3CH=0. As in the propene case, the eclipsed conformer will be favored. However, as can be seen from the diagrams below, the eclipsed form will be favored to a lesser extent in acetaldehyde relative to propene. 

Experimentally, acetaldehyde is known to exist in the eclipsed conformation. It has a methyl rotational barrier of 1.16 kcal/mol94-96 as contrasted with a barrier of 2.00 kcal/mol in the case of propene. 

A number of investigators have examined the ease of isomerization about the formal carbon-carbon bond in imine anions. " Bergbreiter and Newcomb determined an energy barrier of approximately 17 kcal mol" for rotation about this bond in both the lithium imine anions derived from cyclohexyl- and f-butyl-amine addition to acetaldehyde. The barrier for rotation for the corresponding anion derived... 

Ab initio MO calculations (HF/4-31G) indicate that the eclipsed conformation of acetaldehyde is about l.lkcal more stable than the staggered conformation. Provide an explanation of this effect in terms of MO theory. Construct a qualitative MO diagram and point out the significant differences that favor the eclipsed conformation. Identify the interactions that are stabilizing and those that are destabilizing. Identify other factors that need to be considered to analyze the origin of the rotational barrier. 

A number of aldehydes have been studied by NMR and found to have analogous rotameric compositions. Only when the substituent is exceptionally stericaliy demanding, as in (CH3)3CCH2CHO, does the hydrogen-eclipsed conformation become more stable. The barrier heights are somewhat smaller than for the analogous 1-alkenes. For acetaldehyde, the rotational barrier is 1.1 kcal/mol, versus 2.0 kcal/mol for propene. ... 

Figures 17A and 17B (p. 183) show energy as a function of rotation for a series of 1-substituted acetaldehydes, with 6 = 0° in the syn conformation and 6 = 180° in the anti conformation. The calculations were done using the PM3 method. Figure 17A for a vacuum, whereas Fig. 17B is for a solvent cavity with a dielectric constant of 4." The table gives the calculated barriers. Discuss the following aspects (a) rationalize the order Br > Cl > F for syn conformers (b) rationalize the shift to favor the am. conformation in the more polar environment. 

Absolute activity, 12, 13 Absolute intensity, 192 Acetaldehyde barrier height of internal rotation, 378, 382, 383, 388 Acetonitrile, in clathrate, 20... 

Platinum-cobalt alloy, enthalpy of formation, 144 Polarizability, of carbon, 75 of hydrogen molecule, 65, 75 and ionization potential data, 70 Polyamide, 181 Poly butadiene, 170, 181 Polydispersed systems, 183 Polyfunctional polymer, 178 Polymerization, of butadiene, 163 of solid acetaldehyde, 163 of vinyl monomers, 154 Polymers, star-shaped, 183 Polymethyl methacrylate, 180 Polystyrene, 172 Polystyril carbanions, 154 Potential barriers of internal rotation, 368, 374... 

Consider acetaldehyde, CH3CHO. Figure 8.3 shows a for the methyl and CHO protons to differ substantially, so that Vq ax Jax- The low barrier to internal rotation causes condition (1) to be satisfied. Hence the first-order analysis of the preceding paragraphs is applicable. We have an A3X case and the spectrum consists of a doublet (from the methyl protons) whose lines are of equal intensity and a quartet (from the CHO proton) whose lines have the intensity ratios 1 3 3 1 the doublet and quartet are well separated and show the same splitting (Fig. 8.9). 

Abraham and Pople (1960) measured the temperature dependence of the spin coupling constant in acetaldehyde and propionaldehyde. They were able to show that in the most stable forms the carbonyl group eclipses the methyl group in propionaldehyde and a hydrogen atom in acetaldehyde. Powles and Strange (1962) made more extensive measurements of JHH by the spin-echo method and assumed an earlier value of 1-16 kcal mole-1 for the energy barrier to internal rotation. 

Most barriers to internal rotation turn out to be repulsive dominant. Such is the case for methanol, methylamine, propane, propene, hydrazine, and, as has been seen, ethane and ethyl fluoride. Attractive dominant barriers are indicated for acetaldehyde, hydroxylamine, and hydrogen peroxide. 

There is a net positive overlap of +0.0108 in the 0,H-eclipsed conformation between the methyl hydrogens and the carbonyl oxygen, which is reduced significantly to +0.0009 in the less stable H,H-eclipsed conformation. These bonding interactions indicated by overlap populations translate into attractive energy components of sufficient magnitude to cause the rotational energy barrier of acetaldehyde to be attractive dominant. 

The tables may be used to calculate barrier heights in cases where appropriate spectroscopic data are not available, but where experimental values of heat capacity or entropy are known at one or more temperatures. The calorimetrically determined value of the barrier height may then be used in conjunction with the tables to calculate internal rotation contributions to thermodynamic properties over an extended temperature range. Examples of this procedure include calculations for ethane," propene, acetaldehyde, buta-1,2-diene, acetic acid, hexafluoro-ethane, 3-methylthiophen, and 2-methylthiophen. Where spectroscopic values of the barrier height have subsequently been determined, satisfactory agreement has been obtained with the earlier calorimetric values. The agreement between calorimetric (8.16 kJ mol ) and subsequent micro-wave [(8.28 0.07) kJ mol ] values of the barrier height in propene... 

The internal dynamics of the methyl group immensely complicates the spectroscopy of these molecules. Of course, this aspect of the problem also provides much of the spectroscopic interest. When the methyl hydrogens of acetaldehyde oscillate around the CC axis, they experience forces arising from the CHO frame of the molecule which vary sinusoidally. As a result, the potential function for internal rotation can be represented by a cosine function in which the crest to trough distance measures the height of the potential barrier. Since the energy barrier to methyl rotation is low in acetaldehyde, the internal motion is one of hindered internal rotation, rather than torsional oscillation. 

The barrier to internal rotation of the methyl group of acetaldehyde was initially determined by Kilb et al. from an analysis of the microwave spectrum. Since then, the values of the potential constants have been continually revised. Recently, Crighton and Bell combined the available microwave and infrared data with ab initio theory and refined the torsional parameters. Table 17 collects their internal rotation parameters. 

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