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Alkanes
Alkanes are fully saturated hydrocarbons
- hydrocarbons have only Cs and Hs- saturated = all single bonds (max. # of Hs)
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 AppliedChemistry)
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Alkanes lin
e-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
CH3CH
2CH
2CH
3CH3CH2CH3 CH3CH2CH2CH2CH3CH3CH2CH2CH3CH3CH2CH3 CH3CH2CH2CH2CH3
PentaneButanePropane PentaneButanePropane
Condensedstructural
formula
Line-angleformula
Ball-and-stick model
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Naming Straight-Chain Alkanes and Alkyl Groups
The names of all alkanes end in -ane
The number of carbons in a straight-chain alkane isindicated 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 thealkane name to -yl
Example: CH3-CH2- = ethyl
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Classification of Carbons in Alkanes
Carbons can be classified by how many other Cs
are attached to them:-No Cs = methyl CH4
- 1 C = primary (1) CH3-CH3
- 2 Cs = secondary (2) CH3-CH2-CH3
- 3 Cs = tertiary (3) CH3-CH(CH3)2
- 4 Cs = quaternary (4) C(CH3)4
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Naming Branched Alkanes
First find the longest chain of Cs (parent)
Number the Cs in parent chain (begin at endnearest 1rst branch point)
Identify substituents and number by point ofattachment 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 isused and a number is given for each substituent
Example:
CH3-CH2-C(CH3)3 = 2,2-dimethylbutane
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IUPAC Names1. 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 ofcarbon 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
CH3CHCH3
CH3
2-Methylpropane
12
3
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IUPAC Names
4. If there is one substituent, number the
parent chain from the end that gives the
substituent the lower number
CH3
CH3CH
2CH
2CHCH
35
2-Methylpentane(not 4-methylpentane)
12
34
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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
CH3CH2CHCH2CHCH3
CH3 CH3
2,4-Dimethylhexane(not 3,5-dimethylhexane)
12
3
45
6
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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 positionson opposite ends of the parent chain, give the substituent
of lower alphabetical order the lower number
CH3CH
2CHCH
2CHCH
2CH
3
CH3
CH2CH
3
12
34
56
7
3-Ethyl-5-methylheptane
(not 3-methyl-5-ethylheptane)
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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, andthen insert these prefixes
CH2CH3
CH3CCH2CHCH2CH3
CH3
CH34-Ethyl-2,2-dimethylhexane
(not 2,2-dimethyl-4-ethylhexane)
2 34
5
6
1
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Constitutional Isomerism
Constitutional isomers:compounds that have thesame molecular formula but different structural
formulas
for the molecular formulas CH4, C2H6, and C3H8, only one
structural formula is possible; there are no constitutionalisomers for these molecular formulas
for the molecular formula C4H10, two constitutional
isomers are possible
CH3CH2CH2CH3 CH3CHCH3
CH3
Butane
(bp -0.5C)
2-Methylpropane
(bp -11.6C)
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Haloalkanes
Haloalkanes have one or more halogens replacinghydrogen on an alkane
The halogens are numbered and named assubstituents
- F = fluoro
- Cl = chloro
- Br = bromo- I = iodo
If more than one halogen is present, they arenamed in alphabetical order
Example:
CH3-CH(Br)-CH(Cl)-CH3 = 2-bromo-3-chlorobutane
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Cycloalkanes
Carbons can also bond together to form rings
Rings with only Cs, Hs and single bonds arecalled cycloalkanes
Cycloalkanes have the general formula CnH2n
The smallest is cyclopropane (C3H6)
- cyclopropane is an unstable molecule
- its forced to have bond angles of 120betweenthe Cs, while they would normally be 109 (thecarbons each have 4 electrons groups and should
be tetrahedral)
The only cycloalkanes with little or no strain arecyclopentane (C5H10) and cyclohexane (C6H12)
Most cycloalkanes are not flat because they prefertetrahedral geometry
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Cis and Trans Isomers
Because cycloalkanes do not have free rotationaround the carbons, it matters on which side of thering 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 arecalled Trans
Cis and Trans isomers are stereoisomers; theyhave the same molecular formula, and the atoms
are connected in the same order but are arranged ina different spacial orientation
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Cyclohexane
HH
H
HH
H
(a) Ball-and-stick modelshowing all 12 hydrogens
axis through thecen ter of the rin g
H H
H
H
H
H
(b) The s ix equatorialC-H bonds (c) The six axialC-H bonds
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Cyclohexane
the more stable conformation of a substituted
cyclohexane ring has substituent group(s)
equatorial rather than axial
CH3
Equatorial methylcyclohexane
CH3
Axial methylcyclohexane
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Physical Properties of Alkanes and Cycloalkanes
Alkanes are nonpolar and are not soluble in water
They have low melting and boiling points due totheir 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 underhigh heat and pressure, to obtain more gasoline
Alkanes are used extensively as fuels of various
types (for heating, cooking, driving, etc.)
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Reactions of Alkanes and Cycloalkanes
Because they are nonpolar, and their covalentbonds are strong, alkanes and cycloalkanes are
relatively unreactive
The two types of reactions that they do undergoare combustion and halogenation
In combustion reactions, alkanes react withoxygen to form carbon dioxide, water and heat
CH4 + 2O2 CO2 + 2H2O + Heat
In halogenation reactions, halogens replace one or
more Hs on an alkane (a substitution reaction)CH4 + Cl2 (+ light or heat) CH3Cl + HCl
Mechanism of Alkane Halogenation
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Mechanism of Alkane Halogenation These halogenations are radical reactions
Radicals are molecules (or atoms) that have one or more unpairedelectrons (the half-headed arrows represent movement of single electrons
1. Initiation:
H C H
H
H
Cl Cl + Light Cl + Cl
2. Propagation: + ClH C
H
H
+ H-Cl
H C
H
H
+ Cl ClH C Cl
H
H
+ Cl
3. Termination: H C
H
H
+ Cl H C Cl
H
H