Axial chirality

Have a look at the structures (a) and (b). They are the stereoisomers of laballenic acid, with (a) is naturally occurring in plants of the Lamiaceae family. What kind of stereoisomers are they?

(a)	(b)

(−)-laballenic acid (trivial) (5M)-octadeca-5,6-dienoic acid (substitutive, PIN) (5Ra)-octadeca-5,6-dienoic acid (substitutive) (+)-laballenic acid (trivial) (5P)-octadeca-5,6-dienoic acid (substitutive, PIN) (5Sa)-octadeca-5,6-dienoic acid (substitutive)

If there was just one double bond in the middle of the molecule, we’ll be dealing with cis/trans isomerism. But we have two cumulative double bonds, which makes our molecules chiral, even though there are no chiral atoms. Why?

cis and trans

What’s the difference between the structures (a) and (b)?

(a)	(b)

(2Z)-but-2-ene (PIN) cis-but-2-ene (2E)-but-2-ene (PIN) trans-but-2-ene

Enantiomers

Have a look at the structures (a) and (b).

(a)	(b)

(+)-amphetamine (trivial) d-amphetamine (trivial) dextroamphetamine (trivial) dexamfetamine (INN) (2S)-1-phenylpropan-2-amine (substitutive) (−)-amphetamine (trivial) l-amphetamine (trivial) levoamphetamine (trivial) levamfetamine (INN) (2R)-1-phenylpropan-2-amine (substitutive)

Polyhedral symbols and configuration indices

Although structural descriptors such as we’ve seen in the names of boron hydrides, for example catena or closo, provide information on atomic connectivity, they tell us little or nothing about the geometry of the molecule.

Have a look at the structures (a) and (b):

(a)	(b)

(SPY-5)-pentaoxidotungstate(4−) (additive) (TBPY-5)-pentaoxidotungstate(4−) (additive)

Both of them can be named additively pentaoxidotungstate(4−). Yet, as you can see, they have very different shapes.

Boron hydride nomenclature

Can we expand the parent hydride naming philosophy much beyond organic chemistry? Not going too far, let’s have a peek at carbon’s immediate neighbour in the periodic table, boron.

(a)

BH3 borane (preselected name) boron trihydride (binary) trihydridoboron (additive)

The mononuclear hydride (a) is systematically named ‘borane’ while neutral boron hydrides as a class are called boranes.

λ-convention

The whole edifice of substitutive nomenclature is based on concept of parent structures, most importantly parent hydrides. Implicit in parent hydrides are the valencies, or bonding numbers, of non-hydrogen atoms. The standard bonding numbers of neutral atoms in parent hydrides are given thus [1]:

Bonding number n	3*	4	3	2	1
	B	C	N	O	F
	Al	Si	P	S	Cl
	Ga	Ge	As	Se	Br
	In	Sn	Sb	Te	I
	Tl	Pb	Bi	Po	At

These bonding numbers correspond to the number of hydrogen atoms in mononuclear hydrides for elements of Group 13 to Group 17.

Ah, if we only had to name compounds containing just carbon, hydrogen and oxygen. As soon as we move beyond, the trouble starts. Consider phosphorus. We know from school chemistry that this element has valencies of 3 and 5. For trivalent phosphorus compounds we can create substitutive names based on the parent hydride PH₃, phosphane (a). But how to deal with pentavalent phosphorus?

(a)	(b)