Chirality Stopwatch Graph Home Investigating chirality This is

Chirality Stopwatch Graph Home

Investigating chirality • This is a model of a molecule of the amino acid alanine. • It can exist in two forms, which are mirror images of each other. • Its central carbon atom has four different groups attached to it. Stopwatch Graph Home

Chirality • A carbon atom with four different groups attached to it is referred to as a chiral centre. • The molecule is chiral, it has no plane of symmetry. • The two mirror image forms of a chiral molecule are called enantiomers. Stopwatch Graph Home

l l l An optically active compound can rotate the plane of polarised light as it passes through a solution of the compound. Optical isomers are enantiomers. Enantiomers are non-superimpossable mirror images of each other. l Stopwatch Graph Home

l Diastereoisomers are not mirror images of each other. Stopwatch Graph Home

What’s a Racemic Mixture? Count the number of left gloves and right gloves. l 6 left and 6 right gloves, correct? Stopwatch Graph Home

l l l I count 8 right gloves, 4 left gloves. So there’s a slight excess of right gloves here. Stopwatch Graph Home

l l ONLY right hand gloves here. 12 right gloves, zero left gloves Stopwatch Graph Home

l l l Gloves are chiral objects. That is, they lack an internal plane of symmetry. Left gloves and right gloves are mirror images of each other, but they can’t be superimposed. In chemistry, there’s a word we have to describe a pair of nonsuperimposable mirror images – they’re called enantiomers. In the first drawing, we have an equal number of left and right gloves (i. e. enantiomers). This is called a racemic mixture of enantiomers. In the second drawing, we have an excess of right gloves compared to left gloves. In a situtation like this we can say we have an “enantiomeric excess” of gloves, or alternatively, the mixture is “enantioenriched” in the right-hand glove. [We can also calculate the “excess” here: the mixture is 66% right and 33% left – so we have a 33% “excess” of the right-hand enantiomer]. In the third drawing, we have only right-hand gloves. This is said to be an “enantiomerically pure” mixture of gloves, since we have only one enantiomer present. Stopwatch Graph Home

Alanine This is what a molecule of alanine looks like if we draw it on paper. O C H 3 C OH CH NH 2 Stopwatch Graph Home

Naming enantiomers • We need a way of identifying which form of a molecule is which. • There are different systems for naming molecules with a chiral centre. • Biologists tend to use L and D to label amino acids, as this makes most sense in a biological context. • The standard system used by chemists for all molecules with a chiral centre is the R/S system. • To identify whether a molecule is in the R form or the S form we use the Cahn–Ingold–Prelog rules. Stopwatch Graph Home

Identifying a molecule using the Cahn–Ingold–Prelog rules 1. Look at the carbon atom that is highlighted here, it has four different groups attached to it. You need to assign priorities to each of these four groups. 2. Look at the atoms directly attached to this carbon. The higher the atomic number, the higher the priority. H 3 C 3. If two atoms have the same atomic number, you need to look at the atoms attached to these atoms and compare their atomic numbers. O OH C C H NH 2 Stopwatch Graph Home

Identifying a molecule using the Cahn–Ingold–Prelog rules 4. Number the groups so that the highest priority is 1 and the lowest priority is 4. 5. Imagine that you are viewing the molecule so that you are looking at the highlighted carbon with the lowest priority group (4) pointing away from you and then trace around the other groups from highest (1) downwards (i. e. 1, 2, 3). O 2 OH C 4 3 H 3 C H C 1 NH 2 Stopwatch Graph Home

Identifying a molecule using the Cahn–Ingold–Prelog rules • If you traced out a clockwise circle, then the molecule is the R form (R for rectus, or right). • If you went anticlockwise it is the S form (S for sinister, Latin for left) 3 1 C R 2 2 1 C S H 3 C O OH C C H NH 2 3 Stopwatch Graph Home

Identifying a molecule using the Cahn–Ingold–Prelog rules • What form is this? 3 2 1 This is the R form Stopwatch Graph Home

Chirality in nicotine The structures of (S)- and (R)-nicotine. The chiral carbon is highlighted by the red asterisk. In (S)-nicotine, the hydrogen on this carbon is pointing towards us, whereas in (R)-nicotine it is pointing away from us. These two structures are non-superimposable mirror images and thus different from each other in a chiral environment. Article on nicotine Stopwatch Graph Home

Dopamine or meth-amphetamine Methamphetamine has a chiral center, meaning that the molecule contains a carbon atom attached to four different atoms or groups at about 104 o from each other. In addition, the molecule has only one chiral center. Chirality in Breaking Bad Stopwatch Graph Home

Naproxen: One enantiomer is used to treat arthritis pain, but the other causes liver poisoning with no analgesic effect. Chirality Stopwatch Graph Home

THE END Stopwatch Graph Home