unit 2 – alkanes and chemical reactions
DESCRIPTION
Unit 2 – Alkanes and Chemical Reactions. Structure and Stereochemistry of Alkanes Nomenclature of alkanes and cycloalkanes Physical Properties Conformational Analysis The Study of Chemical Reactions Kinetics and Thermodynamic Quantities Free Radical Halogenation - PowerPoint PPT PresentationTRANSCRIPT
Unit 2 – Alkanes and Chemical Reactions
Structure and Stereochemistry of Alkanes Nomenclature of alkanes and
cycloalkanes Physical Properties Conformational Analysis
The Study of Chemical Reactions Kinetics and Thermodynamic Quantities Free Radical Halogenation Reactive Intermediates and Transition
States
Hydrocarbons The simplest organic compounds are the
hydrocarbons: organic compounds that contain only
carbon and hydrogen
four general types: alkanes alkenes alkynes aromatic hydrocarbons
Hydrocarbons Alkanes are often called saturated
hydrocarbons they contain the maximum number of
hydrogen atoms per carbon atom.
Alkenes, alkynes, and aromatic hydrocarbons are called unsaturated hydrocarbons they contain fewer H atoms than an
alkane with the same number of carbon atomsH C CH
HH
H
HH C C HC C
H
H
H
H
Alkanes You must know the names and formulas
for the 10 simplest alkanes:
CH4 methaneCH3CH3 ethaneCH3CH2CH3 propaneCH3CH2CH2CH3 butaneCH3CH2CH2CH2CH3 pentaneCH3CH2CH2CH2CH2CH3 hexane
Alkanes You must know the names and
formulas for the 10 simplest alkanes:
CH3CH2CH2CH2CH2CH2CH3 heptaneCH3CH2CH2CH2CH2CH2CH2CH3 octaneCH3CH2CH2CH2CH2CH2CH2CH2CH3 nonaneCH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 decane
Alkanes The alkanes form a homologous series
with a general molecular formula of CnH(2n+2)
Homologous series: a series of compounds in which each
member differs from the next member by a constant unit
Alkanes differ from each other by -CH2-
AlkanesExample: Which of the following are alkanes:
C2H6, C3H6, C5H12, C4H8
Example: What is the formula for an alkane with 12 carbons?
Alkanes The previous alkanes are straight-chain
alkanes: all of the carbon atoms are joined in a
continuous chain also called “normal” alkanes (n-alkanes)
Alkanes containing 4 or more carbons can also form branched alkanes one or more of the carbon atoms form a
“branch” or side-chain off of the main chain
Alkanes An example of a straight chain alkane:
C5H12 pentane
Examples of branched alkanes:
C5H12
2-methylbutane
2,2-dimethylpropane
CH3CHCH2CH3
CH3
CH3CH2CH2CH2CH3
CH3CCH3
CH3
CH3
Alkanes The three structures shown previously for
C5H12 are structural (constitutional) isomers: compounds with the same molecular
formula but different bonding arrangements
Structural isomers have different properties: different melting points different boiling points often different chemical reactivity
Alkane Nomenclature Organic compounds can be named either
using common names or IUPAC names.
CH3CH2CH2CH2CH3
CH3CHCH2CH3
CH3CCH3
CH3
CH3
CH3
CH3CH2CH2CH2CH3
CH3CHCH2CH3
CH3CCH3
CH3
CH3
CH3
CH3CH2CH2CH2CH3
CH3CHCH2CH3
CH3CCH3
CH3
CH3
CH3
pentane
2-methylbutane or
isopentane
2,2-dimethylpropane or
neopentane
Alkane Nomenclature
CH3 CHCH3
R
The common name for any alkane containing a CH3 group on the second carbon in the chain is “isoalkane.”
“iso”
Isohexane (6 C total)
Isobutane
(4 C total)
CH3CHCH3
CH2CH2CH3
CH3CHCH3
CH3
Alkane Nomenclature Most of the time, organic chemists use
the IUPAC names for organic compounds.
LEARN THE RULES FOR EACH CLASS OF COMPOUNDS WE DISCUSS.
Alkane Nomenclature To name an alkane:
Find the longest continuous chain of carbon atoms and use the name of that chain as the base name of the compound: the longest chain is often NOT written in a straight line
Base name: Base name: heptaneheptaneCH3CHCH2CH3
CH2CH2CH2CH3
123
4 5 6 7
Alkane Nomenclature Number the carbon atoms in the longest
chain starting at the end of the chain closest to a substituent a group attached to the main chain that
has taken the place of a hydrogen atom on the main chain
A substituentA substituent CH3CHCH2CH3CH2CH2CH2CH3
123
4 5 6 7
Alkane Nomenclature Name and give the location of each
substituent. Common substituents:
Halo group a halogen atom “Halo” groups are named using “halo”:
Cl chloro Br bromo I iodo F fluoro
Nitro group -NO2
Alkane Nomenclature Common substituents:
alkyl group A group that is formed by removing an H atom from an alkane
the alkyl group attaches to the main chain at the carbon that has lost its H
CH3CH2CHCH2CH2CH3CH2CH3
correct
CH3CH2CHCH2CH2CH3CH3CH2
incorrect
Alkane Nomenclature Alkyl groups are named by replacing the “ane”
ending of the parent alkane with the “yl” ending.
CH4 H CH
Hmethyl
CH3CH2 ethylCH3CH3
CH3CH2CH3
CH3CH2CH2 propyl
CH3CHCH3 isopropyl
Alkane Nomenclature
CH3CH2CH2CH3
CH3CH2CH2CH2 butyl
CH3CH2CHCH3 sec- butylbutane
CH3CHCH3
CH3
CH3CHCH2
CH3
CH3CCH3
CH3
isobutyl
t- butylisobutane
Alkane Nomenclature
CH3CCH3
CH3
CH3
CH3CCH2
CH3
CH3
neopentyl
neopentane
phenyl
benzene
Alkane Nomenclature Complex alkyl substituents
Use the longest alkyl chain of the substituent as the base name of the substituent
Number the substituent chain with the “head carbon” as carbon 1
List substituents on the alkyl chain with the appropriate numbers
Use parentheses around the name of the group
CHCHCH3
CH2CH3
CH3
a (1- ethyl- 2- methylpropyl) group
Alkane Nomenclature
Methyl groupMethyl group
3-methylheptane
Note: Separate numbers from letters using a hyphen. Separate groups of numbers using commas.
CH3CHCH2CH3CH2CH2CH2CH3
123
4 5 6 7
Alkanes Alkane Nomenclature:
When two or more substituents are present, list them in alphabetical order:
isopropyl before methyl t-butyl or sec-butyl before chloro
When more than one of the same substituent is present (i.e. two methyl groups), use prefixes to indicate how many. Give the location of each as well.
Di = two Tri = three Tetra = four Penta = five Hexa = six
Know Know these.these.Note: Ignore these prefixes when alphabetizing.
Alkane Nomenclature Additional rules:
When there are two “longest” chains of equal length, use the chain with the greater number of substituents.
incorrect correct
CH3 CH CH2
CH3
CHCH3
CH CH2 CH3
CHCH3
CH3
CH3 CH CH2
CH3
CHCH3
CH CH2 CH3
CHCH3
CH3
Alkane Nomenclature Additional rules:
If each end of the longest chain has a substituent the same distance from the end, start with the end nearer to the second substituent.
correct incorrect3-chloro-2,5-dimethylhexane
CH3CHCHCH2CHCH3
CH3
Cl CH31 2
3 4 5 6CH3CHCHCH2CHCH3
CH3
Cl CH3
1234
5
6
AlkanesExample: Name the following compounds:
CH3CHCHCH3CH2CH3
Br
AlkanesExample: Name the following compound:
CH3CHCHCH2CHCH3C(CH3)3
CH(CH3)2
CH2CH3
CH3CCH2CH2NO2CH2CH3
CHCH3
CH3
Alkanes You must also be able to write the structure
of an alkane when given the IUPAC name.
Identify the main chain and draw the carbons in it.
Identify the substituents (type and #) and attach them to the appropriate carbon atoms on the main chain.
Add hydrogen atoms to the carbons to make a total of 4 bonds to each carbon
AlkanesExample: Draw the structure for the following compounds:
3, 3-dimethylpentane
4-sec-butyl-2-methyloctane
1,2-dichloro-3-methylheptane
2-nitropropane
Alkane NomenclatureExample: Draw the structural isomers of hexane (C6H14). Name each isomer.
Use a systematic approach to draw structural isomers:
Draw the unbranched isomer for the first structure.
For other structures, remove 1 or more carbons (and/or functional groups) from the unbranched isomer and reposition to make unique compounds
Types of Carbon Atoms Primary carbon (1o)
a carbon bonded to one other carbon
Secondary carbon (2o) a carbon bonded to two other carbons
Tertiary carbon (3o) a carbon bonded to three other carbons
C
C
C
CH
HH
C CH
HCCC
H
C
C
C
CH
HH
C CH
HCCC
H
C
C
C
CH
HH
C CH
HCCC
H
C
C
C
CH
HH
C CH
HCCC
H
Physical Properties Solubility
Alkanes are nonpolar hydrophobic
do not dissolve in water soluble in nonpolar or weakly polar organic solvents
Density: varies from ~0.5 - ~0.8 g/mL
less dense than water (1.0 g/mL) Alkanes float on water
Physical Properties Boiling Point
In general, boiling point increases as the molecular weight of the alkane increases
larger molecules have greater surface area and higher London dispersion forces of attraction
must be overcome for vaporization and boiling to occur
Physical Properties Boiling Point (cont)
Given the same total number of carbon atoms:
BP (branched) < BP (n-alkane)
Branched alkanes are more compact. less surface area
smaller London dispersion forces lower BP
Physical Properties Melting Points:
In general, melting point increases as MW increases
irregular, sawtooth pattern
Physical Properties Melting Point:
Alkanes with odd number of carbons have lower than expected melting points (compared to the previous alkane with an even number of carbons)
Even # carbons better packing in solid structure
higher MP Odd # carbons
do not pack as well lower MP
Physical Properties Melting Points:
Given the same total number of carbon atoms:
MP (branched) > MP (n-alkane) branched alkanes have more compact structure
better packing higher MP
Sources & Uses of Alkanes Alkanes are derived primarily from
petroleum and petroleum by-products:
Refining via fractional distillation gives useful mixtures of alkanes: C2 - C4 liquified petroleum
gas C4 - C9 gasoline C8 - C16 kerosene C10 - C18 diesel C16+ heavy/mineral oil
Reactions of Alkanes Catalytic Cracking:
converts alkanes into more valuable mixtures of smaller alkanes and alkenes
alkane smaller alkanes + alkenes
C12H26
SiO2 or Al2O3
catalyst
SiO2 +
Reactions of Alkanes Hydrocracking:
converts higher alkanes into shorter alkanes and eliminates N and S impurities
Alkane shorter alkanes
C12H26
H2, SiO2 +
H2, SiO2 or Al2O3
catalyst
Reactions of Alkanes Combustion:
a rapid, exothermic redox reaction that converts hydrocarbons into carbon dioxide and water
alkane + O2 CO2 + H2O
2 C6H14 + 19 O2 12 CO2 + 14 H2O
(unbalanced)
Reactions of Alkanes Halogenation:
a reaction in which a halogen atom is substituted for a hydrogen atom on an alkane
alkane + X2 mixture of alkyl halides
CH4 + Cl2 CH3Cl + CH2Cl2 + CHCl3 + CCl4
or h
h
unbalanced
Conformations of Alkanes•The simplest alkane, CH4, is perfectly
tetrahedral:•bond angle = 109.5•C-H bond length = 1.09 A•free rotation of the C-H bond
Conformations of Alkanes Ethane:
Two carbons overlapping sp3 hybrid orbitals form a sigma bond
Conformations of Alkanes The two methyl groups are relatively free to
rotate about the sigma bond between the two carbon atoms sigma bond maintains its overlap at all times
The different arrangements formed by rotation around a single bond are called conformations.
Conformer: a specific conformation a “conformational isomer”
Conformations of Alkanes Conformers are often drawn using
Newman projections: a way of drawing a molecule looking
straight down the bond connecting two carbon atoms
front carbon atom is represented by three lines joined together in a Y shape
back carbon is represented by a circle with three bonds pointing out from it
H
HHH
HH
Conformations of Alkanes
C
C
HH
HH
H
H
CH
HHH
HH
C
C
HH
HH
H
H
CH
HHH
HH3-D structure
of one conformer of ethane
View from this end
Newman projection
=
Conformations of Alkanes An infinite number of conformations are
possible for ethane and higher alkanes. The dihedral angle () can have an
infinite number of values angle between the C-H bonds on the front and back carbons
C
C
HH
HH
H
H
CH
HHH
HH
Conformations of Alkanes
Molecules are constantly rotating through all possible conformations.
Important conformations for ethane:
Eclipsed conformation
Staggered conformation
Skew conformation
Conformations of Alkanes The conformation of ethane changes
constantly at room temperature.
Conformations may have different energies. Lowest energy conformer is most favored. Highest energy conformer is least favored.
Conformational analysis: the study of the energies of different
conformations helps predict which conformation are
favored and which reaction may occur
Conformations of Alkanes Staggered conformation of ethane:
lowest energy most favored electron clouds in the C - H bonds separated as much as possible
Eclipsed conformation of ethane: highest energy least favored
electron clouds of C - H bonds are closest together
Conformations of Alkanes As ethane rotates from the staggered
conformation towards the eclipsed conformation:
potential energy increases due to torsional strain
resistance to rotation or twisting about a bond
Conformations of Alkanes
Conformation of Alkanes Butane:
4 carbon chain held together by end-to-end overlap of sp3 hybrid orbitals on the carbon atoms
tetrahedral geometry around each carbon
free rotation about any C - C bond many conformers of differing energies
are possible
Newman projections of butane are drawn by looking down the central C2 - C3 bond.
Conformations of Alkanes
Totally eclipsed
(0o)Gauche
(60o)Eclipsed
(120o)Anti
(180o)
Conformation of Alkanes Totally eclipsed conformer of butane:
highest energy due to steric hinderance between the methyl groups
Steric hinderance: interference between two bulky groups
that are close enough together so that their electron clouds repel each other
Conformations of Alkanes
Conformations of Alkanes Eclipsed conformer of butane:
second highest in energy due to repulsion of the methyl group on one carbon and the hydrogen atom on the other
All staggered conformers (gauche and anti) of butane are lower in energy than any of the eclipsed conformers.
Anti conformer of butane: lowest energy because methyl groups are
furthest apart
Conformations of Alkanes