Alkanes: Structure, Nomenclature, and Hybridization, Slides of Biochemistry

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Lecture 4

Alkanes

Structure and Nomenclature

Alkanes and Alkane Isomers

• Alkanes: Compounds with C-C single bonds and C-H bonds only (no functional groups)
• (^) Connecting carbons can lead to large or small molecules
• (^) General formula C n

H

2n+ where the number of C’s is n

• (^) Alkanes are saturated with hydrogen (no more can be added
• (^) They are also called aliphatic compounds

Name These Molecules

Decane

HYBRIDISATION OF ORBITALSHYBRIDISATION OF ORBITALS The electronic configuration of a carbon atom is 1s 2 2s 2 2p 2 (^1) 1s 2 2s 2p

Alkane Isomers

• CH

4 = methane, C 2

H

6 = ethane, C 3

H

8 = propane

• (^) The molecular formula of an alkane with more than three carbons can give more than one structure - C 4 (butane) = butane and isobutane - C 5 (pentane) = pentane, 2-methylbutane, and 2,2- dimethylpropane
• (^) Alkanes with C’s connected to no more than 2 other C’s are straight-chain or normal alkanes
• (^) Alkanes with one or more C’s connected to 3 or 4 C’s are branched-chain alkanes

Constitutional Isomers

• (^) Isomers that differ in how their atoms are arranged in chains are called constitutional isomers
• (^) Compounds other than alkanes can be constitutional isomers of one another
• (^) They must have the same molecular formula to be isomers

Names of Small Hydrocarbons

No. of Carbons Formula Name (C n

H

2 n +

1 Methane CH 4 2 Ethane C 2

H

6 3 Propane C 3

H

8 4 Butane C 4

H

10 5 Pentane C 5

H

12 6 Hexane C 6

H

14 7 Heptane C 7

H

16 8 Octane C 8

H

18

Alkyl Groups

• (^) Alkyl group – remove one H from an alkane

(a part of a structure)

• (^) General abbreviation “R” (for Radical, an

incomplete species or the “rest” of the

molecule)

• (^) Name: replace - ane ending of alkane with - yl

ending

– – CH

3 is “methyl” (from methane)

• (^) – CH 2

CH

3 is “ethyl” from ethane

Naming Alkanes

• (^) Compounds are given systematic names by a process that uses - (^) Prefix-Parent-Suffix
• (^) Follows specific rules
  • (^) Named as longest possible chain
  • (^) Carbons in that chain are numbered in sequence
  • (^) substituents are numbered at their point of attachment
  • (^) Compound name is one word
  • (^) Complex substituents are named as compounds would be

Properties of Alkanes

• (^) Called paraffins (low affinity compounds)

because they do not react as most chemicals

• (^) They will burn in a flame, producing carbon

dioxide, water, and heat

• (^) They react with Cl 2

in the presence of light to

replace H’s with Cl’s (not controlled)

Cycloalkanes

• (^) Cycloalkanes are alkanes that have carbon atoms that form a ring (called alicyclic compounds)
• (^) Simple cycloalkanes rings of CH2 units, (CH2) n , or C n H2 n
• (^) Structure is shown as a regular polygon with the number of vertices equal to the number of C’s (a projection of the actual structure) cyclopropane cyclopentane cyclohexane cyclobutane

Complex Cycloalkanes

• (^) Naturally occurring materials contain cycloalkane structures
• (^) Examples: chrysanthemic acid (cyclopropane), prostaglandins (cyclopentane), steroids (cyclohexanes and cyclopentane)

Naming Cycloalkanes

• (^) Count the number of carbon atoms in the ring and the number in the largest substituent chain. If the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane
• For an alkyl- or halo-substituted cycloalkane, start at a point of attachment as C1 and number the substituents on the ring so that the second substituent has as low a number as possible.
• (^) Number the substituents and write the name

Cis-Trans Isomerism in Cycloalkanes

• (^) Rotation about C-C bonds in cycloalkanes is limited by the ring structure
• (^) Rings have two “faces” and substituents are labeled as to their relative facial positions
• (^) There are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite sides (trans)