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Chapter 2 HYDROCARBON FRAMEWORKS. ALKANES

Chapter 2 HYDROCARBON FRAMEWORKS. ALKANES. Chapter 2: Hydrocarbon Frameworks. Alkanes. LINEAR COMBINATION OF ATOMIC ORBITALS (LCAO). Atomic orbitals can combine via addition or subtraction (hence, linear combination) and form: MOLECULAR ORBITALS, if the starting atomic orbitals

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Chapter 2 HYDROCARBON FRAMEWORKS. ALKANES

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  1. Chapter 2HYDROCARBON FRAMEWORKS. ALKANES Chapter 2: Hydrocarbon Frameworks. Alkanes

  2. LINEAR COMBINATION OF ATOMIC ORBITALS (LCAO) • Atomic orbitals can combine via addition or subtraction (hence, linear combination) and form: • MOLECULAR ORBITALS, if the starting atomic orbitals • belong to different atoms • HYBRID ORBITALS, if the starting atomic orbitals • belong to the same atom • GENERAL RULE:The number of new orbitals (molecular or hybrid) is ALWAYS EQUAL to the number of starting orbitals Chapter 2: Hydrocarbon Frameworks. Alkanes

  3. s- ands*- molecular orbitals Chapter 2: Hydrocarbon Frameworks. Alkanes

  4. s-BONDS AND p-BONDS • s-bonds can form by the combination of: • Two s-orbitals: b. Two p-orbitals: c. One s- and one p-orbital: Chapter 2: Hydrocarbon Frameworks. Alkanes

  5. s-BONDS AND p-BONDS 2. p-bonds form via interaction of p-orbitals only. Chapter 2: Hydrocarbon Frameworks. Alkanes

  6. s-BONDS AND p-BONDS • IMPORTANT NOTES: • s-orbitals, due to their spherical symmetry, cannot participate in the formation of p-orbitals. • The first bond between any pair of atoms is • ALWAYS a s-bond. • 3. Only ONE s-bond can be formed between any pair of two atoms. The rest (if any) are p-bonds. Chapter 2: Hydrocarbon Frameworks. Alkanes

  7. PREDICTIONS BASED ON ELECTRONIC STRUCTURE OF CARBON • There is one 2s and three 2p orbitals, to form four bonds total. It would be therefore expected that one of the bonds would be different from the other three. • The 2s orbital is spherical (non-directional). The 2p orbitals are perpendicular to each other. One would therefore expect that the newly formed bonds would be at a 90o angle. • BOTH PREDICTIONS ARE FALSE!!!!! Chapter 2: Hydrocarbon Frameworks. Alkanes

  8. HYBRIDIZATION AND MOLECULAR SHAPES 121.7o 109.5o 116.6o ethylene planar methane tetrahedral 180.0o acetylene linear Chapter 2: Hydrocarbon Frameworks. Alkanes

  9. SEVERAL WAYS TO REPRESENT METHANE Chapter 2: Hydrocarbon Frameworks. Alkanes

  10. 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 2: Hydrocarbon Frameworks. Alkanes

  11. RULE 1 RULE 2 Chapter 2: Hydrocarbon Frameworks. Alkanes

  12. 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 (n-alkyl groups); b) iso-substituents; Both a) and b) have primary (1o) carbon atoms attached to the main chain. They are primary alkyl groups. c) sec-substituents; The carbon atom attached to the main chain is secondary (2o). The alkyl group is a secondary alkyl group. d) tert-substituents (t-substituents); The carbon atom attached to the main chain is tertiary (3o). The alkyl group is a tertiary alkyl group. Chapter 2: Hydrocarbon Frameworks. Alkanes

  13. 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.Do not alphabetize these prefixes!! • 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. The carbon directly bound to the main chain must be in the chain selected for the base of the name of the substituent! It is always number 1. IUPAC RULES (Contd.) Chapter 2: Hydrocarbon Frameworks. Alkanes

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