Amides, Amino Acids, and Chirality: Naming, Production, and Optical Isomerism, Study notes of Chemistry

Download Amides, Amino Acids, and Chirality: Naming, Production, and Optical Isomerism and more Study notes Chemistry in PDF only on Docsity! OCR Chemistry A H432 Amides, Amino Acids & Chirality Page 1 Amides, Amino acids and Chirality Naming of Amides The amide functional group consists of a carbonyl group bonded to the nitrogen of an amine. Like amines, amides are classified by the number of carbons attached to the nitrogen. A primary amide is therefore one in which both of the other atoms bonded to the nitrogen are hydrogens: -CONH2 e.g. CH3CH2CONH2 propanamide (note that the carbon of the carbonyl group is counted in the longest chain, and that they stem is propan- not propyl-) A secondary amide is a linking group (biologists may refer to this as a peptide link) with the form –CONH- forming the link. e.g. CH3CH2CONHCH3 N-methylpropanamide A tertiary amide has the carbonyl group and two other carbon atoms attached to the e.g. HCON(C2H5)2 N,N-diethylmethanamide Note that this type of hybrid between a displayed and a structural formula is OK if you are only required to draw a molecule unambiguously, but NOT if the type of formula is specified in the question as displayed. Production of amides Amides are produced by the reaction of acyl chlorides (or acid anhydrides) with ammonia or amines. The ammonia or amide acts as a nucleophile, donating the lone pair on the nitrogen to form a dative bond to the δ+ carbon of the acyl chloride. Primary amides are produced in the reaction with ammonia: e.g. CH3COCl + 2NH3 ! CH3CONH2 + NH4Cl ethanoyl chloride ethanamide Secondary amides are produced in the reaction with a primary amine: e.g. CH3CH2COCl + 2CH3NH2 ! CH3CH2CONHCH3 + CH3 +NH3Cl- propanoyl chloride methylamine N-methylpropanamide methylammonium chloride C C C O N H H H H H H H O N C NH O CH2 C3H CH2 CH3 OCR Chemistry A H432 Amides, Amino Acids & Chirality Page 2 Amino Acids Amino acids are the building blocks for biological molecules called peptides, and for proteins (polypeptides). The body has 20 different amino acids from which to assemble proteins. Structure An amino acid contains the functional groups –NH2 and –COOH (amine and carboxylic acid). In an ∝-amino acid (such as the 20 the body uses to make proteins) the –NH2 and –COOH groups are both bonded to the same carbon atom. The general formula of an ∝-amino acid can therefore be written as: or RCH(NH2)COOH The R-group is usually an alkyl group but can contain –OH, -SH, -COOH or –NH2 groups. Example of ∝-amino acids: glycine glutamic acid alanine serine (aminoethanoic acid) (2-aminopropanoic acid) (2-aminopentanedioic acid) (2-amino-3-hydroxypropanoic acid) EXTENSION MATERIAL – NOT ON SPECIFICATION Zwitterions The acidic carboxylic acid group can donate a proton to the basic amine group. The result is an internal salt (a molecule having both a positive and a negative charge on different parts of the same molecule) known as a zwitterion. A zwitterion has no overall charge. e.g. ! OCR Chemistry A H432 Amides, Amino Acids & Chirality Page 5 Identifying optical isomers We find a chiral centre anywhere the four groups attached to a carbon atom are different. This sounds straight forward, but can be made difficult in two ways: i) when the molecule is presented skeletally (and so hydrogen atoms are hidden) e.g. in 2-chloro-5-ethyloctane there are two chiral carbons, indicated with *. Hint: it is easier to see chiral carbons if you re-draw the molecule displayed. ii) when the molecule is large or contains rings - the points of difference can be quite a distance from the chiral carbon, and the whole of the molecule has to be considered. e.g. this molecule only has one chiral centre, indicated with *. You need to be sure you understand both why this carbon is chiral, and why the others aren’t – especially the carbon two to the right of the chiral one. Practice: Which of the following molecules has chiral centers, and where are they: CH3CH2CH2CH2CH2OH - no CH3CH2CH(NH2)CH3 - yes 3rd C from the left is chiral CH3CHClCH3 - no CH3CH(OH)Br - yes 2nd C from left is chiral - yes, four of them shown with * Cl ** * * * * * OCR Chemistry A H432 Amides, Amino Acids & Chirality Page 6 N OH OH Drawing the optical isomers • Structures must be drawn around a correctly-drawn 3D tetrahedral carbon centre • Each enantiomer is then drawn as the mirror image of the other one • Structural formulae may be used for attached groups if convenient (and unless told otherwise in the question) • Use a vertical dotted line to show the mirror plane between the two molecules • Remember to draw the connecting bonds to the correct atoms on the four groups Significance of Optical Isomerism Optical activity is important in biological systems where frequently only one of the optical isomers is biologically active – only one of the optical isomers will interact with an enzyme due to the specific geometry of the receptor sites on enzymes – we call this being stereospecific. This can result in them having different sensory effects or medical effects. e.g. leucine – an amino acid with R = -CH2CH(CH3)2 One optical isomer tastes sweet, the other tastes bitter and is used as a food additive. e.g. Thalidomide (1954) • prescribed to prevent morning-sickness in pregnant women • drug was later found to be chiral – only one enantiomer had the required thereputic effect • the drug was not marketed in the US because the FDA demanded further testing before licensing, and during this process the activity of the other enantiomer was discovered • the other optical isomer led to deformities in developing babies – some 10,000 were affected in Europe e.g. Seldane – one of the first antihistamines • used to relieve hayfever symptoms • chiral with one enantiomer having the required thereputic effect • after testing and licensing, it was found that the “inactive” isomer caused a potentially fatal heart condition in some patients OCR Chemistry A H432 Amides, Amino Acids & Chirality Page 7 COOH e.g. Ibuprofen • one active optical isomer controls pain effectively by blocking messages to the brain and reducing swelling and inflammation • the other isomer is inactive, but is fortuitously converted into the active isomer in the body, so the whole dose is active