Allylic strain

Allylic strain (also known as A^1,3 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. 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. 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 caused by the double bond. Organic chemists capitalize on the rigidity resulting from allylic strain for use in asymmetric reactions.

Quantifying Allylic Strain Energy

The "strain energy" of a molecule is a quantity that is difficult to precisely define, so the meaning of this term can easily vary depending on one's interpretation. 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 ΔH_eq 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 used due to the greater ease of acquisition.

Strain

Strain may refer to:

People

See also

Strain (injury)

A strain is an injury to a muscle in which the muscle fibers tear as a result of overstretching. A strain is also colloquially known as a pulled muscle or torn muscle. The equivalent injury to a ligament is a sprain.

Signs and symptoms

Typical symptoms of a strain include localized stiffness, discoloration and bruising around the strained muscle.

Causes

Strains are a result of muscular fiber tears due to over stretching; they can range from mild annoyance to very painful. Although strains are not restricted to athletes and can happen while doing everyday tasks, people who play sports are more at risk of developing a strain.

Treatment

The first-line treatment for a muscular strain in the acute phase include five steps commonly known as P.R.I.C.E.

Strain (biology)

In biology, a strain is a low-level taxonomic rank used in three related ways, usually at the infraspecific level (within a species).

Microbiology or virology

A strain is a genetic variant or subtype of a micro-organism (e.g., virus or bacterium or fungus). For example, a "flu strain" is a certain biological form of the influenza or "flu" virus. Compare clade.

Plants

The term has no official ranking status in botany; the term refers to the collective descendants produced from a common ancestor that share a uniform morphological or physiological character. A strain is a designated group of offspring that are either descended from a modified plant (produced by conventional breeding or by biotechnological means), or which result from genetic mutation.

As an example, some rice strains are made by inserting new genetic material into a rice plant, all the descendants of the genetically modified rice plant are a strain with a unique genetic code that is passed on to later generations; the strain designation, which is normally a number or a formal name, covers all the plants that descend from the originally modified plant. The rice plants in the strain can be bred to other rice strains or cultivars, and if desirable plants are produced, these are further bred to stabilize the desirable traits; the stabilized plants that can be propagated and "come true" (remain identical to the parent plant) are given a cultivar name and released into production to be used by farmers.