Alkanes are hydrocarbons that contain only single bonds. Their general molecular formula is C(n)H (2n+2).
Constitutional isomers have the same molecular formula, but their atoms are linked differently.
Alkanes are named by determining the number of carbons in their parent hydrocarbon: the longest continuous chain.
Substituents are listed in alphabetical order, with a number to designate their position on the chain.
When there is only a substituent, the substituent gets the lowest possible number; when there is only a functional group suffix, the functional group suffix gets the lowest possible number; when there is both a functional group suffix and a substituent, the functional group suffix gets the lowest possible number.
The functional group is the center of reactivity in a molecule. Alkyl halidesand ethersare named as substituted alkanes. Alcohols and amines are named using a functional group suffix. Systematic names can contain numbers; common names never do.
A compound can have more than one name, but a name must specify only one compound. Whether alkyl halides or alcohols are primary, secondary, or tertiary depends on whether the X (halogen) or OH group is bonded to a primary, secondary, or tertiary carbon.
A primary carbon is bonded to one carbon, a secondary carbon is bonded to two carbons, and a tertiary carbon is bonded to three carbons. Whether amines are primary, secondary, or tertiary depends on the number of alkyl groups bonded to the nitrogen.
Compounds with four alkyl groups bonded to nitrogen are called quaternary ammonium salts. The oxygen of an alcohol has the same geometry it has in water; the nitrogen of an amine has the same geometry it has in ammonia.
The greater the attractive forces between molecules (van der Waals forces, dipole–dipole interactions, hydrogen bonds) the higher is the boiling point of the compound.
A hydrogen bond is an interaction between a hydrogen bonded to an O, N, or F and the lone pair of an O, N, or F in another molecule. The boiling point increases with increasing molecular weight of the homolog.
Branching lowers the boiling point.
Polarizability indicates the ease with which an electron cloud can be distorted: Larger atoms are more polarizable.
Polar compounds dissolve in polar solvents, and nonpolar compounds dissolve in nonpolar solvents.
The interaction between a solvent and a molecule or an ion dissolved in that solvent is called solvation.
The oxygen of an alcohol or an ether can drag about three or four carbons into solution in water.
Rotation about a bond results in two extreme conformations that rapidly interconvert: staggered and eclipsed. A staggered conformer is more stable than an eclipsed conformer because of torsional strain— repulsion between pairs of bonding electrons. There can be two different staggered conformers: The anti conformer is more stable than the gauche conformer because of steric strain—repulsion between the electron clouds of atoms or groups.
The steric strain in a gauche conformer is called a gauche interaction. Five- and six-membered rings are more stable than three- and four-membered rings because of angle strain that results when bond angles deviate from the ideal bond angle of 109.5°.
In a process called ring flip, cyclohexane rapidly interconverts between two stable chair conformations. Bonds that are axial in one chair conformer are equatorialin the other and vice versa. The chair conformer with a substituent in the equatorial position is more stable, because there is more room in an equatorial position. A substituent in an axial position experiences unfavorable 1,3-diaxial interactions. In the case of disubstituted cyclohexanes,the more stable conformer will have its larger substituent in the equatorial position. A cis isomer has its two substituents on the same side of the ring; a trans isomer has its substituents on opposite sides of the ring. Cis and trans isomers are called geometric isomers or cis–trans isomers. Cyclohexane rings are more stable if they are trans fused than cis fused.
from - Organic chemistry - Bruice - Chapter 2 summary
Nessun commento:
Posta un commento