Flip ch2 groups in chemdoodle4/12/2023 ![]() What causes this extra energy? Ring strain Both bond angle strain and torsional strain contribute to the overall strain found in cyclopropane. (n x 157.4) kcal/mol When we measure the actual heats of combustion of these molecules, they turn out to be generally larger in magnitude. Requisite value for ΔH°comb(CH2) When averaged over a large number of alkanes, this value approaches 157.4 kcal/mol, our requisite value for ΔH°comb(CH2)! ΔH°comb of the cycloalkanes With this requisite value for ΔH°comb(CH2), we can calculate the expected ΔH°comb of the cycloalkanes (CH2)n ΔH°comb pattern ΔH°comb increases by about the same amount with each successive member of the homologous series (about 157 kcal/mol for each additional CH2 moiety). If it's not, it might signal presence of strain. ![]() If we had an experimental number for the contribution of a "strain-free" CH2 fragment to the ΔH°comb, the corresponding ΔH°comb should be multiples of this number. How do we compare cycloalkane and alkane stability? We can rewrite the formula for cycloalkanes as (CH2)n instead of (CnH2n). Such a direct comparison is flawed since the empirical formula of cycloalkanes (CnH2n) differs from that of normal alkanes (CnH2n+2) by two hydrogens. ![]() Stability of cycloalkanes To find out whether there is something special about the stability of cycloalkanes, we could compare their heats of combustion with those of the analogous straight-chain alkanes. The heats of combustion of the cycloalkanes reveal the presence of ring strain. The heat content of an alkane can be estimated by measuring its heat of combustion. What is the value of measuring heat of combustion? One measure of the stability of a molecule is its heat content. As ring size increases, strain _ As ring size increases, strain diminishes cyclohexane can be assembled without distortion or strain. The C - C - C bond angles in cyclopropane (60°) and cyclobutane (90°) differ considerably from the tetrahedral value of 109.5°. ![]() ring strain The reason for ring strain lies in the tetrahedral carbon model. Because of the possibility of structural and cis-trans isomerism, a variety of structural possibilities exist in substituted cycloalkanes For example, there are eight isomeric bromomethylcyclohexanes (three are shown below), all with different and distinct physical and chemical properties. conformational isomerism conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds Why are cis and trans isomers not conformational isomers? Cis and trans isomers can be interconverted only by breaking bonds. Constitutional (or structural) isomers Constitutional (or structural) isomers are compounds that have the same molecular formula and different connectivity. Stereoisomers Stereoisomers are isomeric molecules that have the same molecular formula and sequence of bonded atoms (constitution), but differ in the three-dimensional orientations of their atoms in space. The positions of any remaining hydrogens are not always shown. Hashed-wedged line structures We can use hashed-wedged line structures to depict the three-dimensional arrangement of substituted cycloalkanes. Substituents on opposite faces are called trans. In the other isomer, they are on opposite faces. Substituents on the same face are called cis. Disubstituted cycloalkanes can be stereoisomers In one isomer, the two substituents are positioned on the same face (side) of the ring. This phenomenon has been ascribed to differences in crystal packing forces between the two series. In comparing lower cycloalkanes possessing an odd number of carbons with those having an even number, we find a pronounced alternation in their melting points. ![]() These differences are due in large part to increased London interactions of the relatively more rigid and more symmetric cyclic systems. The properties of the cycloalkanes differ from those of their straight-chain analogs Compared with the corresponding straight-chain alkanes, the cycloalkanes have higher boiling and melting points as well as higher densities. For example, methylcyclopropane (not cyclopropylmethane) and cyclobutylcyclohexane (not cyclohexylclycobutane). The smaller unit is treated as a substituent to the larger one. Substituted cycloalkanes are sometimes named as cycloalkyl derivatives. Cycloalkyl radicals Radicals derived from cycloalkanes by abstraction of a hydrogen atom ![]()
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