Alkanes

1. Alkanes • Alkanes are fully saturated hydrocarbons - hydrocarbons have only C’s and H’s - saturated = all single bonds (max. # of H’s) • Alkanes have the general formula CnH2n+2 - example: C4H10 • They can be straight-chained or branched - example: CH3CH2CH2CH3 vs. (CH3)2CHCH3 • Alkanes, and all other organic compounds, are named according the the IUPAC system (International Union of Pure and Applied Chemistry)

2. Alkanes • line-angle formula: • a line represents a carbon-carbon bond and an angle represents a carbon atom • a line ending in space represents a -CH3 group • hydrogen atoms are not shown in line-angle formulas

4. Naming Straight-Chain Alkanes and Alkyl Groups • The names of all alkanes end in -ane • The number of carbons in a straight-chain alkane is indicated by putting a prefix before the -ane • Examples: CH4 = methane CH3-CH3 = ethane CH3-CH2-CH3 = propane • In branched alkanes the substituents (groups attached to the carbon chain) are called alkyl groups • An alkyl group = an alkane with one H removed • Alkyl groups are named by changing the ending of the alkane name to -yl • Example: CH3-CH2- = ethyl

7. Classification of Carbons in Alkanes • Carbons can be classified by how many other C’s are attached to them: - No C’s = methyl CH4 - 1 C = primary (1) CH3-CH3 - 2 C’s = secondary (2) CH3-CH2-CH3 - 3 C’s = tertiary (3) CH3-CH(CH3)2 - 4 C’s = quaternary (4) C(CH3)4

8. Naming Branched Alkanes • First find the longest chain of C’s (parent) • Number the C’s in parent chain (begin at end nearest 1rst branch point) • Identify substituents and number by point of attachment to parent chain • Write full name Example: CH3-CH(CH3)-CH2-CH3 = 2-methylbutane • If there is more than one of a substituent a prefix is used and a number is given for each substituent • Example: • CH3-CH2-C(CH3)3 = 2,2-dimethylbutane

9. IUPAC Names 1. The name for an alkane with an unbranched chain of carbon atoms consists of a prefix showing the number of carbon atoms and the ending -ane 2. For branched-chain alkanes, longest chain of carbon atoms is the parent chain and its name is the root name 3. Name and number each substituent on the parent chain; use a hyphen to connect the number to the name

10. IUPAC Names 4. If there is one substituent, number the parent chain from the end that gives the substituent the lower number

11. IUPAC Names 5. If the same substituent occurs more than once, • number the parent chain from the end that gives the lower number to the substituent encountered first • indicate the number of times the substituent occurs by a prefix di-, tri-, tetra-, penta-, hexa-, and so on • use a comma to separate position numbers

12. IUPAC Names 6. If there are two or more different substituents • list them in alphabetical order • number the chain from the end that gives the lower number to the substituent encountered first • if there are different substituents in equivalent positions on opposite ends of the parent chain, give the substituent of lower alphabetical order the lower number

13. IUPAC Names 7. Do not include the prefixes di-, tri-, tetra-, and so on, or the hyphenated prefixes sec- and tert- in alphabetizing; • alphabetize the names of substituents first, and then insert these prefixes

14. Conformations of Alkanes • Because alkanes have only single bonds they have free rotation about those bonds • This allows for different spatial arrangements of the atoms called conformations • Conformations are not the same as isomers • Some conformations are more stable than others, so the compound spends more time in those • For ethane(CH3-CH3): - when all H’s are lined up as you sight down the C-C bond, called eclipsed, this is highest energy - when none of the H’s are lined up, called staggered, this is lowest energy

17. Constitutional Isomerism • Constitutional isomers:compounds that have the same molecular formula but different structural formulas • for the molecular formulas CH4, C2H6, and C3H8, only one structural formula is possible; there are no constitutional isomers for these molecular formulas • for the molecular formula C4H10, two constitutional isomers are possible

18. Haloalkanes • Haloalkanes have one or more halogens replacing hydrogen on an alkane • The halogens are numbered and named as substituents - F = fluoro - Cl = chloro - Br = bromo - I = iodo • If more than one halogen is present, they are named in alphabetical order • Example: CH3-CH(Br)-CH(Cl)-CH3 = 2-bromo-3-chlorobutane

19. Cycloalkanes • Carbons can also bond together to form rings • Rings with only C’s, H’s and single bonds are called cycloalkanes • Cycloalkanes have the general formula CnH2n • The smallest is cyclopropane (C3H6) - cyclopropane is an unstable molecule - it’s forced to have bond angles of 120between the C’s, while they would normally be 109 (the carbons each have 4 electrons groups and should be tetrahedral) • The only cycloalkanes with little or no strain are cyclopentane (C5H10) and cyclohexane (C6H12) • Most cycloalkanes are not flat because they prefer tetrahedral geometry

21. Naming Cycloalkanes • Cycloalkanes are named by adding cyclo- to the parent alkane name • Substituents are numbered when there is more than one of them • They are numbered starting at the one that gives the others the lowest numbers • Or, they are numbered alphabetically when there are only two, or both directions are equal

22. Cis and Trans Isomers • Because cycloalkanes do not have free rotation around the carbons, it matters on which side of the ring a substituent is relative to other substituents • Two substituents on the same side (top or bottom) of the ring are called Cis • Two substituents on opposite sides of the ring are called Trans • Cis and Trans isomers are stereoisomers; they have the same molecular formula, and the atoms are connected in the same order but are arranged in a different spacial orientation

24. Cyclohexane

25. Cyclohexane • the more stable conformation of a substituted cyclohexane ring has substituent group(s) equatorial rather than axial

26. Physical Properties of Alkanes and Cycloalkanes • Alkanes are nonpolar and are not soluble in water • They have low melting and boiling points due to their weak intermolecular forces (dispersion) • They are also less dense than water • Alkanes are mostly obtained from crude oil - the crude oil is fractionated based on boiling pt. - heavier fractions are often “cracked”, put under high heat and pressure, to obtain more gasoline • Alkanes are used extensively as fuels of various types (for heating, cooking, driving, etc.)

28. Reactions of Alkanes and Cycloalkanes • Because they are nonpolar, and their covalent bonds are strong, alkanes and cycloalkanes are relatively unreactive • The two types of reactions that they do undergo are combustion and halogenation • In combustion reactions, alkanes react with oxygen to form carbon dioxide, water and heat CH4 + 2O2  CO2 + 2H2O + Heat • In halogenation reactions, halogens replace one or more H’s on an alkane (a substitution reaction) CH4 + Cl2 (+ light or heat)  CH3Cl + HCl

29. Mechanism of Alkane Halogenation • These halogenations are radical reactions • Radicals are molecules (or atoms) that have one or more unpaired electrons (the half-headed arrows represent movement of single electrons