Chiral molecules are optically active :
Starting with heptane, the possibility exists that a single skeletoncorresponds to several spatial configurations. In particular, this happens whenever the molecule includes just one so-called chiral carbon, namely a carbon atom bonded to 4 different ligands. In that case, we are faced with achiral compoundwith two different possible configurations which are mirror images of each other (they are calledenantiomers). As it is traversed by a ray of polarized light,a pure enantiomer in fluid form (or in a solution) will rotatethe angle of polarization by a angle proportional to the molar density and the distancetravelled.
Such an optical activity is observed for the following two types of heptane. Each of these has two enantiomers because each has a single chiral atom :
The fact that either of those yields a pair of enantiomers is the reason why there are 11 stereoisomers of heptane for only 9 structural isomers.
It'softenthe case that a molecule with k chiral carbons has 2k stereoisomers. The simplest exception among alkanes is the following octane,featuring two chiral carbons but only 3 (not 4) stereoisomers; a pair ofoptically active enantiomers and one inactive meso compound. (: As the two halves may rotate around the axis ofthe two chiral carbons, the meso isomer is center-symmetric.)
3,4-Dimethylhexane = ( C*H CH3 C2H5 ) 2
A star superscript ( C*) is used to indicate that a given carbon is chiral.
On the limited usefulness of the "chiral carbon" concept :
Spiranes are cycloalkanes which contain two cycles that share a single carbon atom. At that central atom, the two pairsof bonds that define the planes of the two cycles are perpendicular. The simplest example of a spirane is spiroheptane,which consists of 2 carbon quadrilaterals sharing one vertex.
Spiranes can illustrate some of the difficulties associated with chiral carbons in the analysis of delicate cases. For example, consider the following pair of enantiomersfordimethylspiroheptane C9H16
This is clearly a chiral compound because those two mirror images cannot be superposed. Such molecules are sometimes wrongly heralded as havingno chiral carbons. A close examination reveals that this is not the case; the above chiralmolecule does feature 3 chiral carbons (the central carbon and the twocarbons attached to methyl groups).
Indeed, a carbon is chiral whenever it's attached to 4 different ligands. Two chiral ligands that are enantiomers of eachother are different! When two ligands are interconnected by a structurally symmetrical chain,the case may not be obvious to settle. In the case of a carbon attached to a methyl group in the aforementioned molecule ofdimethylspiroheptane, the chain that goes fromone bond to the other and the chain that goes back have different chiralities (otherwise the whole molecule would not be chiral). Both of those carbonsare therefore chiral.
The case of the central atom is even trickier. It belongs to two oriented 4-cycles which are symmetrical but chiral (we may decide to observe from the side of the methyl group and describe unambiguouslya direction as either clockwise or counterclockwise). Some thinking is needed to realize that two identical chiral loops meeting perpendicularly at one point form a chiralconfiguration (the two chirality do not cancel, so to speak). The central carbon is thus chiral as well.
Removing an hydrogen atom from an alkane yields an activechemical entity called an alkyl group (it's eager to combine with some other "free" group,as the two unpaired electrons from both groups tend to form a covalent bond).
As already illustrated above, such groups are commonlynamed after thesimple alkane they are derived from (by removing an hydrogen froma carbon atom at the end of a chain): Methyl, ethyl, propyl, butyl, etc.
-CH3 -C2H5 -C3H7 -C4H9 ...
In the standard nomenclature used to describe "branched" alkanes,the longest carbon chainis used along with the names and numeric positions of the akyl groups borneby carbons on that chain. Symmetries are usually taken advantage of,in order to make the numeric positions as small as possible.
For example, a descriptive name for isobutane is methylpropane: A methyl group attached to the middle atom (position 2) in the 3-chain of propane... The position is not explicited in this case because there'sonly one possibility which does not yield a compound with asimpler name (namely, straight butane ).
Several akyl groups may be attached to the same carbon atom. For example,dimethylpropane properly describes a pentane (also called neopentane) consisting of a central carbon atom attachedto 4 identical methyl groups.
The simplest aromatic compound is benzene C6H6 whose cyclic structure wasfirst proposed in 1865 by August Kekulé (1829-1896)the father of structural chemistry.
A Phenyl -C6H5 (symbol Ph, formerly ) consists of six carbonsin a circle, all bonded to an hydrogen with at most one exception (no exception in the case of benzene, which may be denoted by the formula PhH). All six carbon atoms are equally spaced, regardless of their positions withrespect to the substituent.
Toluene (PhCH3) is also known as phenyl methane. as it consists of a phenyl group and a methy group, the IUAPC recommends the systematic name methylbenzene.
Phenol (PhOH). Also called carbolic acid. It consists of a phenyl group and an hydroxyl group (-OH). As such, it's also known as phenyl hydroxide.
