Figure 2.22 Pa, b shows energy as a function of rotation...

Figure $2.22 \mathrm{~Pa}, \mathrm{~b}$ shows energy as a function of rotation for a series of 2 -substituted acetaldehydes, with $\theta=0^{\circ}$ in the syn conformation and $\theta=180^{\circ}$ in the anti conformation. The calculations were done by the PM3 method. Figure 2.P22a represents the isolated molecule, while Figure 2.P22b represents an elliptical solvent cavity with a dielectric constant of $4.7$, approximating $\mathrm{CHCl}_{3}$. The Table $2.22 \mathrm{P}$ gives the calculated rotational barriers. Discuss the following aspects of the data. (a) Rationalize the $\mathrm{Br}>\mathrm{Cl}>\mathrm{F}$ order of preference for anti conformation in the gas phase; (b) Why does the polar medium shift the equilibrium to favor more of the $s y n$ conformation?

Key Concepts

Conformational Isomerism

This concept refers to the existence of different spatial arrangements of a molecule that can be interconverted by rotation about single bonds. In the case of 2-substituted acetaldehydes, the syn and anti conformations arise from the rotation around the bond connecting the substituent to the acetaldehyde framework. Understanding these conformations is essential as they determine the molecule's overall energy and reactivity, and small changes in bond rotation can lead to significant differences in physical and chemical properties.

Substituent Electronic Effects

Substituents such as halogens can influence molecular conformation through their intrinsic electronic properties, including electronegativity and polarizability. In the gas phase, the order of anti conformation preference (Br > Cl > F) can be rationalized by noting that larger, more polarizable halogens like bromine tend to stabilize the anti conformation via less pronounced dipole–dipole interactions than the strongly electron?withdrawing and less polarizable fluorine. This balance of electronic effects and subtle intramolecular forces determines the relative energies of the conformers.

Steric and Electrostatic Interactions

Conformational preferences are significantly affected by nonbonded steric and electrostatic interactions between different parts of a molecule. In these acetaldehydes, the spatial arrangement of the substituent in the syn versus anti conformations results in different steric clashes and dipole alignments. Minimization of unfavorable steric hindrance and electrostatic repulsion, particularly between the substituent and the carbonyl group, helps explain the observed differences in conformational barriers.

Solvent Polarity Effects

The impact of a polar medium on conformational equilibrium is rooted in solvation effects, where solvents with significant dielectric constants can stabilize polar conformers more effectively. In a polar environment, the syn conformation, typically possessing a higher overall dipole moment due to aligned individual bonds, is stabilized through favorable dipole–solvent interactions. This leads to a shift in equilibrium, making the syn conformation more competitive compared to the gas phase where intrinsic molecular interactions dominate.

Transcript

0:00 Hello everyone.

00:01 Let us see the following question.

00:03 The synchlinical confirmation of 1 .2 dichloroethane.

00:07 1 .2 dichloroethane.

00:19 It is 4 .8 kilojole per mole.

00:26 So higher the energy than anti -periplanar confirmation.

00:30 So this is given by synchlinical confirmation.

00:43 Anti -periplanar confirmation is lower to that of synclinical confirmation.

00:51 The two energy barriers for rotation about cc bond in 1 .2 dichloroputane are given as two other bond rotations are given as 21 .5 kilojol per mole and another is 38 .9 kilojole per mole and another is 38 .9 kilojole per mole.

01:10 These are two other confirmations.

01:13 But these are higher than that of anti -periplanar conformer.

01:18 Now, synclinical conformer is given a rotation barrier of 4 .8 kilojoule per mole.

01:24 Okay? now, anti -periplanar, it is higher than that of anti -periplanar.

01:30 Now, two other confirmations say confirmation a and confirmation b are given with these rotational barrier energy of 21 .5 kilojol per mole and 38 .9 kilojoule per mole.

01:42 Both are higher to that of anti -periplanar confirmation.

01:46 Now, the question a regarding this will be sketch a graph of energy versus angle of rotation about cc bond of dihydral angle of 1 .2 dichloroethane.

01:59 Now we have to draw all the conformers of 1 .2 dichloroethane.

02:03 So we need to sketch a graph.

02:06 Sketch a graph.

02:10 So let us see the answer.

02:13 Now, as we can see here, we have different barriers like, say for example, the first barrier is about the carbon atom and the other carbon atom is exactly eclipsing over each other.

02:26 So this in common word we can call it as eclipsed conformer where two ch3 groups, the normal butane is 1, 2, 3, 4.

02:36 That is this is a ch3 group.

02:38 This is a ch3 group.

02:41 There has been two hydrogen here at this carbon.

02:43 There has been two hydrogen at this carbon.

02:46 So which is represented in newman projection formula like this, where two ch3 are exactly overlapping each other, that is called eclips to conformal.

02:56 So when they eclipse over each other, the two ch3 groups are starrically hindered with each other.

03:02 So, this is called a stertic string...

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