ALKANES
CnH2n+2
Alkanes or CnH2n+2
C3H8
Chemical
Formula
CH3CH2CH3
Condensed Structural
Formula
H H H
H–C–C–C–H
H H H
Structural
Formula
Alkanes: base unit
CH4
CH3CH3
or C2H6
CH3CH2CH3
or C3H8
CH3CH2CH2CH3
or C4H10
CH3CH2CH2CH2CH3
or C5H12
Difference between each is CH2
Properties of Alkanes
• Change systematically with number of C’s
• As number C’s increases, boiling point ↑
– molecules get heavier
Properties of Alkanes
• Low Reactivity
– except readily undergo combustion – fuels
• Nonpolar – Don’t dissolve well in water
• Low mp’s & bp’s
– mp & bp ↑ with ↑ molecular mass
• High vapor pressures
– evaporate easily
Which property is generally
characteristic of an organic compound?
A.
B.
C.
D.
Low melting point
High melting point
Soluble in polar solvents
Insoluble in nonpolar solvents
Correct response = A
Which of the following compounds
has the highest boiling point?
A.
B.
C.
D.
CH4
C2H6
C3H8
C4H10
Correct answer = D
bp depends on strength of forces holding molecules
together [forces ↑as size of molecule ↑]
Naming
Hydrocarbons
Naming straight-chain Alkanes
• Name describes molecule so can
draw it
• All alkanes have the suffix –ane
• prefix tells how many C’s
Prefix
Meth
Eth
Prop
But
Pent
Hex
Hept
Oct
Non
Dec
# of C atoms
1
2
3
4
5
6
7
8
9
10
Formula
CH4
C2H6
C3H8
C4H10
C5H12
Name
Meth + ane
Ethane
Propane
Butane
Pentane
Branched-chain alkanes
Beginning with butane, C4H10, more than 1
way to arrange the atoms
H
H–C–H
H H
H–C–C–C–H
H H H
Branched alkane
- can’t link all C’s
without lifting
pencil off paper
Methyl propane
Isomers
• Compounds with same molecular formula
but different structural arrangement
• more C atoms  more isomers
(more possible ways to arrange them)
Isomers
• Different structures = different
properties
–different chemical & physical properties
A note about isomers --• comparing 2 structural formulas: if can
superimpose them, then not isomers = same
molecule!
• If can rotate or flip one of structural formulas &
superimpose it on the other, they are not isomers
– they are same molecule!
These diagrams
represent the
same molecule
Represents bond to H
These diagrams
represent the
same molecule
These diagrams
represent the
same molecule
Two different
molecules!
Naming
Branched
Hydrocarbons
Naming Branched-Chain Alkanes
• goal of name: describe molecule so can draw it
Naming branched-chain alkanes
• find longest continuous chain (backbone) of C
atoms (Bends don’t count!)
• base name derived from # C’s in longest chain
• branches named first
– Branches named by # C’s
– “branch” name ends in “yl”
• assign #’s to C’s in backbone
• # from end so lowest # given to branch
• more than 1 same type branch:
– use di, tri, tetra
H
H–C–H
H H
1 2
3
H–C–C–C–H
H H H
Longest continuous
chain has 3 carbon
atoms = propane
Branch: 1 carbon – methyl
Branch at C #2:
- can’t be on any other C so
doesn’t need #
Methyl propane
C4H10
CH3CH(CH3)CH3
Longest continuous chain: 6 C’s = hexane
H
6
H–C–H
5
H–C–H
HH H
4
H–C–C–C–C–H
3 2 1
H H
H
H–C–H
H
2-methyl hexane
branch is 1 carbon long – methyl.
branch located at C #2
C7H16
CH3CH(CH3)CH2CH2CH2CH3
A note about branches
• If several branches with same # C’s can
condense name a bit
• 2-methyl 3-methyl pentane becomes
2,3-dimethyl pentane
• every branch must have a #
Longest continuous chain has
6 C’s: hexane
H
-branch 1 C = methyl
-branch located at C #3
H–C–H
H H H
H– C–C–C–C–H
H
H
H–C–H H–C–H
H
H
3-methyl hexane
C7H16
CH3CH2CH(CH3)CH2CH2CH3