Allylic strain (also known as A1,3 strain, 1,3-allylic strain, or A-strain) in organic chemistry is a type of strain energy resulting from the interaction between a substituent on one end of an olefin with an allylic substituent on the other end.[1] If the substituents (R and R') are large enough in size, they can sterically interfere with each other such that one conformer is greatly favored over the other.[2] Allyic strain was first recognized in the literature in 1965 by Johnson and Malhotra. The authors were investigating cyclohexane conformations including endocyclic and exocylic double bonds when they noticed certain conformations were disfavored due to the geometry constraints caud by the double bond.[3] Organic chemists capitalize on the rigidity resulting from allylic strain for u in asymmetric reactions.[2]
confliction
Allylic strain in an olefin.
Quantifying Allylic Strain Energy
The "strain energy" of a molecule is a quantity that is difficult to precily define, so the meaning of this term can easily vary depending on one's interpretation.[4] Instead, an objective way to view the allylic strain of a molecule is through its conformational equilibrium. Comparing the heats of formation of the involved conformers, an overall ΔHeq can be evaluated. This term gives information about the relative stabilities of the involved conformers and the effect allylic strain has on equilibrium. Heats of formation can be determined experimentally though calorimetric studies; however, calculated enthalpies are more commonly ud due to the greater ea of acquisition.[4]
Different methods utizilized to estimate conformational equilibrium enthalpy include: the Westheimer method,[5] the homomorph method,[6] and more simply—using estimated enthalpies of nonbonded interactions within a molecule.[3] Becau all of the methods are approximations, reported strain values for the same molecule can vary and should be ud only to give a general idea of the strain energy.
Olefins
Calculated rotational energies for different conformations of 3-methyl-1-butene.[7] Allylic 1,3-strain is most prevalent in 1c, making this conformation the highest in energy.
麻省理工大学公开课The simplest type of molecules which exhibit allylic strain are olefins. Depending on the substituents, olefins maintain varying degrees of allylic strain. In 3-methyl-1-butene, the interactions between the hydrogen and the two methyl groups in the allylic system cau a change in enthalpy equal to 2 kcal/mol.[7][考研 政治verification needed] As expected, with an increa in substituent size, the equilibrium enthalpies between rotamers also increas. For example, when examining 4-methyl-2-pentene which contains an additional allylic methyl group compared to 3-methyl-1-butene, the enthalpy of rotation for the highest energy conformer increas from 2 kcal/mol to 4 kcal/mol.[7]
ladies and gentlemen四级听力真题下载Cyclic Molecules
Various strain interactions shown in red. (Other hydrogens left off for simplicity)
mythsNonbonded 1,3-diaxial interaction energies are commonly ud to approximate strain energy in cyclic molecules, as values for the interactions are available. By taking the difference in nonbonded interactions for each conformer, the equilibrium enthalpy can be estimated. The strain energy for methylenecyclohexane has been calculated to be 4.5 kcalmol−1 using estimations for 1,3-diaxial strain (0.9 kcalmol−1), methyl/hydrogen allylic strain (1.3kcalmol−1), and methyl/methyl allylic strain (7.6 kcalmol−1) values.[2]
The strain energy in 1,8-dimethylnaphthalene was calculated to be 7.6 kcalmol−1skin是什么意思 and around 12-15 kcalmol−1 for 4,5-dimethyl-phenanthrene.[2] Allylic strain tends to be great
er for cyclic molecules compared to olefins as strain energy increas with increasing rigidity of the system. An in depth summary of allylic strain in six membered rings has been prented in a review by Johnson, F.[2]
nativespeaker1,8-dimethylnaphthalene and 4,5-dimethylphenanthrene
Influencing Factors
Several factors influence the energy penalty associated with the allylic strain. In order to relieve strain caud by interaction between the two methyl groups, the cyclohexanes will often exhibit a boat or twist-boat conformation. The boat conformation tends to be the major conformation to the strain.[2]important The effect of allylic strain on cis alkenes creates a preference for more linear structures.[1]
Substituent Size
