Nomenclature and Conformations of Alkanes and Cycloalkanes: A Comprehensive Guide
Nomenclature and Conformations of Alkanes and Cycloalkanes
Chapter 4 Terms
IUPAC System (Sec 4.3)
A system for naming compounds through naming rules, with the fundamental principle that each different compound should have an unambiguous name; originally developed in 1892 by the International Union of Pure and Applied Chemistry.
Alkyl Halide (Sec 2.5, 4.3E, 6.1)
Compound in which a halogen atom (F, Cl, Br, I) replaces a hydrogen atom of an alkane, with the general formula RX. Also called haloalkane.
Alcohol (Sec 2.6, 4.3F, 11.1)
Molecule containing a hydroxyl group, –OH, bound to a sp3 hybridized carbon atom, with the general formula ROH.
Cycloalkane (Sec 4.4, 4.7, 4.10)
Alkane in which at least some of the carbon atoms are arranged in a ring.
Bicyclic Compound (Sec 4.4B, 4.14)
A molecule with two fused rings that share at least two carbon atoms.
Vinyl Group (Sec 4.5)
The common name for the ethenyl group, H2C=CH─
Allyl Group (Sec 4.5, 13.2)
The common name for the 2-propenyl group, H2C=CHCH2─
Cis-trans Isomers (Sec 1.13B, 4.5, 4.13, 7.2)
Diastereomers that differ in their stereochemistry at adjacent atoms of a double bond or on different atoms of a ring; the cis-isomer has two substituents on the same side of a double bond or ring; the trans-isomer has two substituents on opposite sides of a double bond or ring.
Acetylenic Hydrogen Atom (Sec 4.6, 7.9)
The hydrogen atom that is bound to a carbon atom of a terminal alkyne, RC≡CH, which is slightly acidic with pKa = 25.
Conformations (Sec 4.8)
The temporary molecular shapes that result from rotation about the single bonds in a molecule.
Conformers (Sec 4.8)
Each possible temporary structure that results from rotation about the single bonds in a molecule.
Conformational Analysis (Sec 4.8, 4.9, 4.11, 4.12)
An analysis of the energy changes that occur as a molecule undergoes rotations about its single bonds.
Newman Projection Formula (Sec 4.8)
A means of representing the spatial relationships of groups attached to two atoms of a molecule; when viewing the molecule from one end directly along the bond axis joining the two atoms, bonds that are attached to the front atom are shown as radiating from the center of a circle; and those attached to the rear atom are shown as radiating from the edge of the circle.
Sawhorse Projection (Sec 4.8)
A structural formula indicating the spatial arrangement of bonds.
Staggered Conformation (Sec 4.8)
A temporary orientation of groups around two atoms joined by single bonds, such that the bonds of the back atom exactly bisect the angles formed by the bonds of the front atom in a Newman projection formula.
Dihedral Angle (Sec 4.8)
The angle that is formed whenever two planes meet.
Potential Energy Diagram (Sec 4.8)
A graph of the energy difference between conformations of a molecule.
Eclipsed Conformation (Sec 4.8)
A conformation of a molecule in which the relative position of any two substituents on adjacent atoms directly oppose each other, with a dihedral angle between their bonds of 0°.
Hyperconjugation (Sec 4.8, 6.11B, 10.2B)
The phenomenon of electron delocalization, via orbital overlap from a filled bonding molecular orbital to an adjacent unfilled orbital, which stabilizes the molecule or ion.
Torsional Barrier (Sec 4.8)
The energy barrier to free rotation about single bonds in a molecule.
Torsional Strain (Sec 4.8, 4.10)
The resistance to twisting about a single bond, which is caused by breaking the favorable orbital overlap that stabilizes a conformation by hyperconjugation, and/or by eclipsed atoms coming so close together that they produce Van der Waals repulsions.
Steric Strain (Sec 4.8, 4.12)
The interference between two groups when they come so close together that they produce Van der Waals repulsions.
Van der Waals Repulsions (Sec 4.9 – 4.11)
Repulsive forces between atoms produced when the van der Waals surface of one atom is penetrated by that of another atom, which causes strong repulsive forces between the overlapping electron clouds. Also called repulsive dispersion forces.
Anti Conformation (Sec 4.9)
A staggered conformation of a molecule in which the two largest substituents on adjacent atoms are the maximum distance apart, with a dihedral angle between their bonds of 180°.
Gauche Conformation (Sec 4.9)
A staggered conformation of a molecule in which the two largest substituents on adjacent atoms are a small distance apart, with a dihedral angle between their bonds of 60°.
Stereoisomers (Sec 1.13B, 4.9A, 4.13, 5.1 – 5.16)
Compounds with the same molecular formula and the same connectivity, but are not superposable because they differ in the arrangement of their atoms in three-dimensional space; includes enantiomers and diastereomers, but excludes constitutional isomers.
Conformational Stereoisomers (Sec 4.9A, 5.14A)
Two stereoisomers whose three-dimensional structures differ only due to rotations about single bonds, and they are interconvertible by bond rotations.
Ring Strain (Sec 4.10)
The increased potential energy of the cyclic form of a molecule when compared to its acyclic form, which is caused mostly by angle strain and torsional strain.
Angle Strain (Sec 4.10)
The strain a molecule possesses because its bond angles are distorted from their normal values; measured by the increased potential energy of a molecule (usually cyclic) caused by deformation of a bond angle away from its lowest energy value.
Cyclohexane Conformations (Sec 4.11 – 4.13)
The three-dimensional structures of cyclohexane that result from rotations about its single bonds; the most stable form is the chair conformation, followed by the twist boat, the boat, and the half chair which is the most unstable form.
Chair Conformation (Sec 4.11)
A conformation of cyclohexane that resembles a chair, which is free of angle strain because all C-C-C bond angles are 109.5°, and is free of torsional strain because all bonds are staggered; it is the most stable (lowest energy) conformation of cyclohexane.
Boat Conformation (Sec 4.11)
A conformation of cyclohexane that resembles a boat, with eclipsed bonds along its two sides; the boat conformation is less stable (higher energy) than the chair conformation.
Axial Bond (Sec 4.12)
The six bonds of a cyclohexane ring that are perpendicular to the average plane of the ring, and that alternate up and down around the ring.
Equatorial Bond (Sec 4.12)
The six bonds of a cyclohexane ring that extend from the perimeter of the ring, and that alternate slightly up and slightly down at an angle of about 20° to the average plane of the ring.
1,3-Diaxial Interaction (Sec 4.12)
Repulsive forces produced by steric strain between groups in axial positions on the same side of a cyclohexane ring.
Polycyclic Compound (Sec 4.14)
A molecule with two or more fused rings.
Hydrogenation (Sec 4.16A, 7.3A, 7.13 – 7.15)
An addition reaction in which hydrogen atoms add to a double or triple bond and converts it into a single bond, which is often accomplished through the use of a metal catalyst, such as platinum, palladium, nickel, rhodium, or ruthenium.
Index of Hydrogen Deficiency (IHD) (Sec 4.17)
A measure of the total number of pi bonds and rings that a molecule contains, which equals the difference in the number of pairs of hydrogen atoms between the molecular formula for the compound under study (CnHx ), and the acyclic alkane having the same number of carbons (CnH2n+2 ), according to the equation: IHD = (2n + 2 – x) / 2. For compounds containing halogens, replace each halogen atom with a hydrogen atom. For compounds containing oxygen, simply remove all oxygen atoms. For compounds containing nitrogen, delete one hydrogen atom for each nitrogen atom, then remove all nitrogen atoms. Also called degree of unsaturation, or double-bond equivalence.