Classes of Hydrocarbons

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Classes of Hydrocarbons
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Hydrocarbons
Aromatic
Aliphatic
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Hydrocarbons
Aromatic
Aliphatic
Alkanes
Alkenes
Alkynes
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Hydrocarbons

• Alkanes are hydrocarbons in which all of the
  bonds are single bonds.

Aliphatic
Alkanes
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Hydrocarbons

• Alkenes are hydrocarbons that contain a
  carbon-carbon double bond.

Aliphatic
Alkenes
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Hydrocarbons

• Alkynes are hydrocarbons that contain a
  carbon-carbon triple bond.

Aliphatic
Alkynes
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Hydrocarbons

• The most common aromatic hydrocarbons are those
  that contain a benzene ring.

Aromatic
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Reactive Sites in Hydrocarbons The Functional
Group Concept
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Functional Group

• a structural unit in a molecule responsible for
  itscharacteristic chemical behavior and its
• spectroscopic characteristics

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Alkanes

• functional group is a hydrogen atom
• the reaction that takes place is
• termed a substitution
• one of the hydrogens is substitutedby some other
  atom or group, X

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Alkanes

• functional group is a hydrogen
• the reaction that takes place is substitution
• one of the hydrogens is substituted by some
  other atom or group

Cl2
HCl
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Functional Groups in Hydrocarbons

• alkanes RH
• alkenes RH, double bond
• alkynes RH, triple bond
• aromatics ArH, double bond

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Some Key Functional Groups
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Families of organic compounds and their
functional groups

• Alcohols ROH
• Alkyl halides RX (X F, Cl, Br, I)
• Amines primary amine RNH2
• secondary amine R2NH
• tertiary amine R3N
• Ethers ROR

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Many classes of organic compounds contain a
carbonyl group
R
Carbonyl group
Acyl group
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Many classes of organic compounds contain a
carbonyl group
H
R
Carbonyl group
Aldehyde
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Many classes of organic compounds contain a
carbonyl group
R'
R
Carbonyl group
Ketone
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Many classes of organic compounds contain a
carbonyl group
OH
R
Carbonyl group
Carboxylic acid
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Many classes of organic compounds contain a
carbonyl group
OR'
R
Ester
Carbonyl group
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Many classes of organic compounds contain a
carbonyl group
NH2
R
Amide
Carbonyl group
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General formula for an alkane
CnH2n2
Introduction to Alkanes Methane, CH4 Ethane,
C2H6 Propane, C3H8
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The simplest alkanes

• Methane (CH4) CH4
• Ethane (C2H6) CH3CH3
• Propane (C3H8) CH3CH2CH3

bp -160C
bp -89C
bp -42C
No isomers possible for C1, C2, C3 hydrocarbons
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Isomeric Alkanes The Butanes
C4H10
General formula for any butane
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C4H10

• n-Butane Isobutane
• CH3CH2CH2CH3 (CH3)3CH

bp -0.4C
bp -10.2C
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Higher n-Alkanes Pentane (C5H12) and Beyond
CnH2n2 n gt 4
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CnH2n2 n gt 4
CH3CH2CH2CH2CH3
n-Pentane
CH3CH2CH2CH2CH2CH3
n-Hexane
CH3CH2CH2CH2CH2CH2CH3
n-Heptane
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The C5H12 Isomers
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C5H12
CH3CH2CH2CH2CH3
(CH3)2CHCH2CH3
Isopentane
n-Pentane
(CH3)4C
Neopentane
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How many isomers?

• The number of isomeric alkanes increases as the
  number of carbons increase.
• There is no simple way to predict how many
  isomers there are for a particular molecular
  formula.

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Table 2.3 Number of Constitutionally Isomeric
Alkanes

• CH4 1
• C2H6 1
• C3H8 1
• C4H10 2
• C5H12 3
• C6H14 5
• C7H16 9

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Table 2.3 Number of Constitutionally Isomeric
Alkanes

• CH4 1 C8H18 18
• C2H6 1 C9H20 35
• C3H8 1 C10H22 75
• C4H10 2 C15H32 4,347
• C5H12 3 C20H42 366,319
• C6H14 5 C40H82 62,491,178,805,831
• C7H16 9

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C6H6 Isomers

• How many isomers with the composition
• C6H6 can you draw?

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C6H6 Isomers How many isomers with the
compositionC6H6 can you draw?
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Structure and Bonding in Alkenes
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Structure of Ethylene

• bond angles H-C-H 117
• H-C-C 121
• bond distances CH 110 pm
• CC 134 pm

planar
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Bonding in Ethylene
s
s
s
s
s

• Framework of s bonds
• Each carbon is sp2 hybridized

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Bonding in Ethylene

• Each carbon has a half-filled p orbital

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Bonding in Ethylene

• Side-by-side overlap of half-filled p orbitals
  gives a p bond

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Isomerism in Alkenes
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Isomers
Isomers are different compounds thathave the
same molecular formula (composition).
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Isomers
Stereoisomers
Constitutional isomers
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Isomers
Stereoisomers
Constitutional isomers
same connectivity different arrangementof atoms
in space
different connectivity
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Isomers
Stereoisomers
Constitutional isomers
consider the isomeric alkenes of molecular
formula C4H8
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1-Butene
2-Methylpropene
trans-2-Butene
cis-2-Butene
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1-Butene
2-Methylpropene
Constitutional isomers
cis-2-Butene
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1-Butene
2-Methylpropene
Constitutional isomers
trans-2-Butene
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Stereoisomers
trans-2-Butene
cis-2-Butene
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Molecular Chirality Enantiomers
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Chirality

• A molecule is chiral if its two mirror image
  forms are not superposable upon one another.
• A molecule is achiral if its two mirror image
  forms are superposable.

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Bromochlorofluoromethane is chiral
Cl

• It cannot be superposed point for point on its
  mirror image.

Br
H
F
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Bromochlorofluoromethane is chiral
Cl
Cl
Br
Br
H
H
F
F

• To show nonsuperposability, rotate this model
  180 around a vertical axis.

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Bromochlorofluoromethane is chiral
Cl
Br
Cl
Br
H
H
F
F
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Another look
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Enantiomers
nonsuperposable mirror images are called
enantiomers
and

• are enantiomers with respect to each other

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Isomers
constitutional isomers
stereoisomers
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Isomers
constitutional isomers
stereoisomers
geometric isomers (cis/trans)
enantiomers (optical)
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Chlorodifluoromethaneis achiral
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Chlorodifluoromethaneis achiral

• The two structures are mirror images, but are
  not enantiomers, because they can be superposed
  on each other.

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Symmetry in Achiral Structures
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Symmetry tests for achiral structures

• Any molecule with a plane of symmetrymust be
  achiral.

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Plane of symmetry

• A plane of symmetry bisects a molecule into two
  mirror image halves. Chlorodifluoromethane has
  a plane of symmetry.

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Plane of symmetry

• A plane of symmetry bisects a molecule into two
  mirror image halves. Chlorodifluoromethane has
  a plane of symmetry.

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Plane of symmetry

• A plane of symmetry bisects a molecule into two
  mirror image halves.1-Bromo-1-chloro-2-fluoroeth
  ene has a planeof symmetry.

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Plane of symmetry

• A plane of symmetry bisects a molecule into two
  mirror image halves.1-Bromo-1-chloro-2-fluoroeth
  ene has a planeof symmetry.

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Physical Properties ofAlkanes and Cycloalkanes
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Boiling Points

• increase with increasing number of carbons
• more atoms, more electrons, more opportunities
  for induced dipole-induced dipole forces
• decrease with chain branching
• branched molecules are more compact
  with smaller surface areafewer points of
  contact with other molecules

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Boiling Points

• increase with increasing number of carbons
• more atoms, more electrons, more opportunities
  for induced dipole-induced dipole forces

Heptanebp 98C
Octanebp 125C
Nonanebp 150C
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Boiling Points

• decrease with chain branching
• branched molecules are more compact
  with smaller surface areafewer points of
  contact with other molecules

Octane bp 125C
2-Methylheptane bp 118C
2,2,3,3-Tetramethylbutane bp 107C
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Boiling Points of Alkanes

• governed by strength of intermolecular
  attractive forces
• alkanes are nonpolar, so dipole-dipole and
  dipole-induced dipole forces are absent
• only forces of intermolecular attraction are
  induced dipole-induced dipole forces

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Induced dipole-Induced dipole attractive forces

• two nonpolar molecules
• center of positive charge and center of negative
  charge coincide in each

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Induced dipole-Induced dipole attractive forces

• movement of electrons creates an instantaneous
  dipole in one molecule (left)

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Induced dipole-Induced dipole attractive forces

• temporary dipole in one molecule (left) induces
  a complementary dipole in other molecule (right)

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Induced dipole-Induced dipole attractive forces

• temporary dipole in one molecule (left) induces
  a complementary dipole in other molecule (right)

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Induced dipole-Induced dipole attractive forces

• the result is a small attractive force between
  the two molecules

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Induced dipole-Induced dipole attractive forces

• the result is a small attractive force between
  the two molecules

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Straight chain hydrocarbon Branched hydrocarbon
Fewer intermolecular contacts
Lots of intermolecular contacts