Chapter 3 STRUCTURE AND STEREOCHEMISTRY OF ALKANES Chapter 3: Structure and Stereochemistry of Alkanes

IUPAC RULES 1. Form the base for the compounds’ name. This is the name of the longest continuous carbon chain (main chain) in the molecule. Other groups attached to the main chain are considered as substituents. If two chains of equal length can be found in a particular molecule, use the one that gives a greater number of substituents. 2. Number the main chain, starting with the end, which is nearest a substituent. Chapter 3: Structure and Stereochemistry of Alkanes

RULE 1 RULE 2 Chapter 3: Structure and Stereochemistry of Alkanes

IUPAC RULES (Contd.) 3. The substituents names are derived in the same fashion as the parent hydrocarbons, but instead of suffix –ane, put a suffix –yl. Assign numbers to the substituents matching their positions on the main chain. The substituents also take prefixes, whenever necessary, to differentiate between different isomeric forms. a) n-substituents; b) iso-substituents; Both a) and b) have primary (1 o ) carbon atoms attached to the main chain. c) sec-substituents; The carbon atom attached to the main chain is secondary (2 o ). d) tert-substituents; The carbon atom attached to the main chain is tertiary (3 o ). Chapter 3: Structure and Stereochemistry of Alkanes

IUPAC RULES (Contd.) 4.Write the name as a single word, using hyphens to separate the different prefixes and using commas to separate the numbers. When two or more substituents are present, list the substituents in alphabetical order. If some of them are identical, list them together with a prefix identifying their total number: di- (two), tri- (three), tetra- (four), penta- (five), etc. 5.When a more complex substituent is encountered, one may need, for the naming of the substituent, to apply the entire procedure outlined above, i.e. select a main chain, etc. But as usual, the substituent as whole should always bear the suffix –yl, instead of – ane. Chapter 3: Structure and Stereochemistry of Alkanes

HEATS OF COMBUSTION AND RING STRAIN RING SIZE CYCLO ALKANE MOLAR HEAT OF COMBUSTION PER CH 2 GROUP RING STRAIN PER CH 2 TOTAL RING STRAIN 3Cyclopropane499.8 kcal/mol kcal/mol Cyclobutane Cyclopentane Cyclohexane Cycloheptane Ref. Long-chain alkane Chapter 3: Structure and Stereochemistry of Alkanes

STRAIN ENERGY VS. RING SIZE Chapter 3: Structure and Stereochemistry of Alkanes

TYPES OF STRAIN IN ORGANIC MOLECULES (SUMMARY) 1.Torsional Strain: The strain due to eclipsing of bonds at neighboring carbon atoms. Energy cost – about 1 kcal/mol for C-H --- C-H eclipse, or 1.3 kcal/mol for C-H --- C-CH 3 eclipse. 2.Steric Strain: The strain due to repulsive interactions, when atoms or groups approach each other too closely. Energy cost – about 0.9 kcal/mol for C-CH C-CH 3 gauche interaction. NOTE: The C-CH C-CH 3 eclipsed interaction is a cumulative torsional + steric strain!!! 3.Angle Strain: The strain due to expansion or compression of bond angles. Energy cost – varies, depending on the actual bond angle. Chapter 3: Structure and Stereochemistry of Alkanes