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The Nature of Organic Compounds: Alkanes and Cycloalkanes 11 million known organic compounds Grouped into few dozen families Simplest family: Alkanes Functional Group (FG): A group of atoms within a molecule that has a characteristic chemical behavior. A functional group behaves in nearly the same way in every molecule, regardless of the size and complexity, for example: H H H C C H KMnO4 OH H H CH3 H H H H C C C OH + H C CH3 H KMnO4 OH H H O H H3 C H H H H + H H H3 C H H H H O C C OH H Table 3.1 lists many common functional groups. FGs based on type of Carbon-Carbon Bonds: C Alkane C C Alkene C C Alkyne 1 FGs with Carbon Singly Bonded to an Electronegative Atom: C C Alkyl halide Alcohol Ether C C N C S SH Thiol Amine C O OH X C C Sulfide In all cases, the bonds are polar. C: δ+ and electronegative atom: δ - FGs with a Carbon-Oxygen double bonded (Carbonyl Groups) Present in some of the most important organic compounds. C H C C C OH C C C O O O Aldehyde Carboxylic Acid Ketone O C C C O Ester C N C Cl C C O O Amide Acid Chloride 2 Alkanes and Alkane Isomers Alkanes are also known as Saturated hydrocarbons Only carbon-carbon single bonds C-C Only C and H General formula: CnH2n+2 n = integer CH4, C2H6, C3H8, C8H18, C12H26 • structure: sp3, td • bond angle: 109° Carbons can be: • straight-chain (normal alkanes) H2 H2 C C H3C C H2 CH3 n-pentane • branched (branched-chain alkanes) CH3 CH3 CH H3 C CH C H2 CH3 2,4-Dimethylpentane 3 Isomers: molecules with the same formula, but different structure. • Constitutional isomers (geometrical isomers) • Conformational • Stereoisomers Constitutional Isomers: Have the same formula but a different connection of atoms, for example: C4H 10 CH 3 H 3C CH CH3 H3C 2-Methylpropane (Isobutane) H H C C H H CH 3 Butane Constitutional isomers are not restricted to alkanes, for example: H3C CH 2OH C 2H 6O H3C O CH3 C2H 6O ethanol diethyl ether Problem: Draw structures to meet the following description: a) Four isomers with the formula C 8H 18 (There are many more!) 4 b) Two isomers with the formula C4H8O2 Names of Straight or Normal Alkanes. No of C 1 2 3 4 5 6 7 Name methane ethane propane butane pentane hexane heptane Formula CH4 C2 H6 C3 H8 C4H10 C5H12 C6H14 C7H16 Alkyl -CH3 -CH2CH3 -CH2CH2CH3 - CH2 CH2CH2CH3 -CH2CH2CH2 CH2 CH3 - CH2 CH2CH2CH2CH2 CH3 - CH2 CH2CH2CH2CH2 CH2 CH3 Alkyl name Methyl (Me) Ethyl (Et) Propyl (Pr) Butyl (Bu) pentyl hexyl heptyl n-alkyl groups H3C CH2 CH 3 H3C CH 2 CH2 Propyl Propane branched alkyl groups H 3C CH 2 Propane CH 3 H3 C CH CH 3 isopropyl 5 Carbon atoms can be classified as primary (1°), secondary (2°), tertiary (3°) and quaternary (4°) by the number of alkyl groups attached. H R C R H H R C H R C R H R C H R R R 1o 2o 3o 4o R CH3 H 2C CH CH2 CH2CH3 C CH3 CH3 6 Nomenclature of Branched-Chain Alkanes IUPAC: International Union of Pure Applied Chemistry Prefix Parent Location of substituents Suffix number of carbons FGs Rules: 1) Longest continuous chain of carbons determines the parent name CH 3CH2CH 2CH 2CHCH3 CH3 CH 3CH2CH2 CH 2CHCH3 CH2 CH2 hexane heptane 2) Number the longest chain beginning with the end of the chain nearest the branching. 6 5 4 3 2 1 CH 3CH2CH2CH2CHCH3 CH 3 7 6 5 4 3 CH 3CH2CH 2CH 2CHCH3 2 CH 2 1 CH 2 3) Use the numbers obtained by application of rule 2 to designate the location of substituents. 2-Methylhexane 3-Methylheptane Numbers are separated from words by a dash. 4) When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain. 1 2 3 4 5 6 CH3CHCH2CHCH2CH3 CH3 CH2 CH3 4-Ethyl-2-methylhexane 7 List groups alphabetically. Disregard prefixes such as: di, tri, sec-, tertExample: ethyl precedes dimethyl tert-butyl precedes ethyl but, ethyl precedes isobutyl 5) If two or more substituents are present on the same carbon, use the number twice CH3 1 2 3 4 5 6 CH3CH2CCH2CH2CH3 CH2 CH3 3-Ethyl-3-methylhexane 6) If 2 or more substituents are identical, use prefixes di, tri, tetra, etc. Commas are used to separate numbers from each other: 1 CH 3 3 4 CH 3 3 CH 3-CH-CH-CH-CH 3 CH 3-CH-CH-CH3 2 CH 3 1 4 2 5 CH 3 CH3 2,3-Dimethylbutane 2,3,4-Trimethylpentane 7) When 2 chains of equal length compete for selection as the base, choose the chain with the greater number of substituents. CH3 CH2 CH2 7 6 5 CH3 3 CH3 -CH2 -CH-CH-CH-CH-CH3 4 CH3 2 1 CH3 2,3,5-Trimethyl-4-propylheptane and not 4-sec-Butyl-2-3-dimethylheptane 8 8) When branching first occurs at an equal distance from either end, choose the name that gives the lower number at the first point of difference. 6 5 CH 3 4 2 1 CH 3 -CH-CH2 -CH-CH-CH 3 3 CH 3 CH 3 2,3,5-Trimethylhexane and not 2,4,5-Trimethylhexane 9) If a substituent of the main chain has sub-branching, name the substituent by applying previous rules and begin numbering at the point of attachment. The complex substituent is set off in parenthesis when writing the complete name of the compound. CH3 CH3 9 8 7 6 5 CH3 -CH 2-CH 2-CH 2-CH-CH-CH-CH3 4 CH 2 3 CH2 CH3 CH 3 2 CH CH3 CH-CH-CH3 1 CH3 1 2-Methyl-5- (1,2-dimethylpropyl) nonane 2 3 5- (1,2-dimethylpropyl) For historical reasons, some simpler branched-chain alkyl groups have nonsystematic names and IUPAC accepts these names. CH3-CH-CH3 Isopropyl (i-Pr) CH3 CH3 -CH2-CH-CH 3 CH3 -CH-CH2 CH3 CH3 -C CH3 Sec-Butyl (sec-Bu) Isobutyl CH3 CH3 -CH-CH2-CH 2 CH3 Isopentyl CH3 -C CH2 CH3 Neopentyl tert-Butyl (t-Bu) CH3 CH3 -CH2 -C CH3 tert-Pentyl 9 Examples Properties of Alkanes Physical properties: ♦ The molecules are non-polar or very weakly polar ♦ The forces holding together non-polar molecules are van der Waals forces. These intermolecular forces, which operate only over very small distances, result from induced polarization of the electron clouds in molecules. ♦ Within a family: The larger the molecule the stronger the intermolecular forces. greater number of Carbons > b.p. > m.p. n-alkanes gases <4 C 5-17 C liquids >18 C solids 10 ♦ Branched-chain isomers have lower bp than straight chain isomers >branches <bp >branches: molecule tends to approach a sphere, so < contacts. n-pentane and neo-pentane Chemical properties: ♦ Inertness to many chemicals (also called paraffins = slight affinity) C-C and C-H are strong bonds, resistant to breaking. C and H have almost the same electronegativity, therefore little polarization. ♦ No unshared electrons for acid attack ♦ Characteristic reactions: Halogenation C H + X2 hν C X + HX heat mixture Combustion nC 12H2 + 8O2 5CO2 + 6H2O ∆H = - 845 Kcal/mole 11 Pyrolysis (or cracking) 400-600° Alkane H2 + smaller alkanes Cycloalkanes (or alicyclic compounds) alkanes in which the carbon atoms are arranged in a ring. General formula: C nH2n Properties: Similar to alkanes • Non polar • Chemically inert to most reagents Cycloalkanes have less freedom of rotation around C-C Kinds: monocyclic, bicyclic, polycyclic Nomenclature: A. Monocyclic: Cycloalkanes with only one ring are named by attaching the prefix cyclo to the name of the corresponding alkane with the same number of carbons. Examples: cyclopropane cyclobutane cyclopentane 12 1) Use the cycloalkane name as the parent methylcyclopentane and NOT cyclopentylmethane If the alkane chain contains greater number of carbons than the ring, designate the ring as the substituent. 1-cyclopropyl butane and NOT butylcyclopropane 2) When more than one substituent is present. Number the ring beginning with one substituent in the way that gives the next substituent the lower number possible. 1,3-Dimethylcyclohexane and NOT 1,5-Dimethylcyclohexane 13 B. Bicyclic: Compounds with two rings. The name of the alkane corresponding to the total # of carbon atoms is used as “base name” bridge head H C H2C CH2 CH2 H2C CH2 C H bridge carbon Bicyclo(2.2.1)heptane or norbornane Bridge head carbon: carbon common to both rings Bridge carbon: carbon connecting bridge heads H C H2 C CH2 C H Bicyclo (1.1.0) butane 14 Stereoisomers: Atoms connected in the same order, but different spatial orientation. Example: OH OH OH OH Cis-1,2-cyclopentanediol trans-1,2-cyclopentanediol Do not interconvert without breaking a bond. 15