Pentane interference or ''syn''-pentane interaction is the

steric hindrance Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...

that the two terminal methyl groups experience in one of the chemical conformations of

n-pentane Pentane is an organic compound with the formula C5H12—that is, an alkane with five carbon atoms. The term may refer to any of three structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, pentane means exclusively the '' ...

. The possible conformations are combinations of anti conformations and

gauche conformation In chemistry, conformational isomerism is a form of stereoisomerism in which the isomers can be interconverted just by rotations about formally single bonds (refer to figure on single bond rotation). While any two arrangements of atoms in a mo ...

s and are anti-anti, anti-gauche⁺, gauche⁺ - gauche⁺ and gauche⁺ - gauche⁻ of which the last one is especially energetically unfavorable. In macromolecules such as

polyethylene Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging ( plastic bags, plastic films, geomembranes and containers including b ...

pentane interference occurs between every fifth carbon atom. The 1,3-diaxial interactions of cyclohexane derivatives is a special case of this type of interaction, although there are additional ''gauche'' interactions shared between substituents and the ring in that case. A clear example of the ''syn''-pentane interaction is apparent in the diaxial versus diequatorial heats of formation of ''cis'' 1,3-dialkyl cyclohexanes. Relative to the diequatorial conformer, the diaxial conformer is 2-3 kcal/mol higher in energy than the value that would be expected based on ''gauche'' interactions alone. Pentane interference helps explain molecular geometries in many chemical compounds, product ratios, and purported transition states. One specific type of ''syn''-pentane interaction is known as ''1,3 allylic strain'' or (A^1,3 strain). For instance in certain aldol adducts with 2,6-disubstituted

aryl In organic chemistry, an aryl is any functional group or substituent derived from an aromaticity, aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar ...

groups the molecular geometry has the vicinal hydrogen atoms in an antiperiplanar configuration both in a crystal lattice ( X-ray diffraction) and in

solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Soluti ...

proton ( NMR coupling constants) normally reserved for the most bulky groups i.d. both arenes: The other contributing factor explaining this conformation is reduction in

allylic strain 250 px , right , Allylic strain in an olefin. 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 olef ...

by minimizing the

dihedral angle A dihedral angle is the angle between two intersecting planes or half-planes. In chemistry, it is the clockwise angle between half-planes through two sets of three atoms, having two atoms in common. In solid geometry, it is defined as the un ...

between the arene double bond and the

methine In organic chemistry, a methine group or methine bridge is a trivalent functional group , derived formally from methane. It consists of a carbon atom bound by two single bonds and one double bond, where one of the single bonds is to a hydrogen. ...

proton. Syn-pentane interactions are responsible for the backbone-conformation dependence of protein side chain rotamer frequencies and their mean dihedral angles, which is evident from statistical analysis of protein side-chain rotamers in the

Backbone-dependent rotamer library In biochemistry, a backbone-dependent rotamer library provides the frequencies, mean dihedral angles, and standard deviations of the discrete conformations (known as rotamers) of the amino acid side chains in proteins as a function of the backb ...

References

{{reflist Stereochemistry