Transcript Chapter 4- Alkanes

Chapter 4- Alkanes
Hydrocarbons
• Alkanes• Alkenes• Alkynes-
• Cycloalkanes- alkanes in which all or some of
the carbon atoms are arranged in rings
General Formulas
• Alkanes- CnH2n+2
• Alkenes- CnH2n
• Alkynes- CnH2n-2
• Cycloalkanes- CnH2n
Sources
• Petroleum
• Cracking
– Catalytic Cracking- special catalysts are used to
break larger alkanes, C12 or more, into smaller
ones, C5-10
– Thermal Cracking- same thing except heat is used
instead of catalysts
Gasoline
• 2,2,4-trimethylpentane- “isooctane”
• Heptane
• Octane Rating
Shapes of Alkanes
• All carbons are sp3 hybridized and tetrahedral
in alkanes and cycloalkanes
• Straight Chain- refers to compound being
unbranched, not actually straight. They
contain only primary and secondary carbons
Naming
• Development of the formal naming system only
came about in the late 1800’s.
• Since many organic molecules had already been
discovered, the older names are referred to as
common names
• The formal naming system was created by the
International Union of Pure and Applied Chemistry.
(IUPAC for short)
Names of Straight Chains
• Like learning to count in Organic Chemistry
• Prefix tells number of carbons, -ane suffix
identifies as an Alkane
Naming unbranched alkyl groups
• Remember, remove a H from an alkyl, you
have an alkyl group.
• when the H is removed from the end, it is a
terminal alkyl group, or Unbranched alkyl
group.
• To name it through IUPAC, we drop the –ane
and add -yl
Naming Branched Chains
• Rules:
1) Locate the longest continuous chain of carbons
2) Number the longest chain beginning with the end
nearest the substituent
3) Use the numbers obtained above to designate the
location of every substituent
Note: numbers are separated from letters with a
hyphen.
Naming Branched Chains, cont
4) When 2 or more substituents are present,
give each a number corresponding to its location
on the longest chain
Note: Substituents are listed alphabetically
in front of the parent name
5) When two substituents are on the same
carbon, use the number twice!
Naming Branched Chains, cont
6) When two or more substituents are identical,
indicate this by using numerical prefixes such as
di-, tri-, tetra-, penta-, etc.
Note: numerical prefixes are not included
with alphabetizing.
Note: make sure each substituent has a
number! Multiple numbers are separated by a
comma
Naming Branched Chains, cont
• Most situations can be handled by these 6
fundamental rules
• There are two more rules for more complex
compounds:
Naming Branched Chains, cont
7) When two chains compete for the longest
parent chain, choose the chain with the greater
number of substituents
8) When branching first occurs at equal distance
from either end of the longest chain, choose the
chain that gives the lowest number at the first
point of difference.
Examples
Naming Branched Alkyl Groups
• Some common names are accepted in IUPAC
– Ex.
• The prefixes sec- and tert- are NOT included in
alphabetizing.
• In IUPAC, similar to regular naming except
numbering always begins with the Carbon attached
to the main chain.
Classification of Hydrogens
• Covered previously
• Same as alcohols and alkyl halides
Naming Alkyl Halides
• Alkanes with halogen substituents are named
in IUPAC as haloalkanes
• Cl= Chloro F= Fluoro Br= Bromo I= Iodo
• When the chain bears both halo and alkyl
substituents, number the chain from the end
nearest the 1st substituent regardless if it is a
halo or alkyl
Naming Alkyl Halides
• If the 1st substituent is equal distance from the
end, start with the end with the substituent
with alphabetical precedence
• Some common names still accepted.
• In common naming, they are named as alkyl
halides
Nomenclature of Alcohols
• In IUPAC substitutive nomenclature, a name may
have four parts:
Locants, prefixes, parent name, suffixes
• In general, the numbering of the chain always
begins at the end nearer the group named as a
suffix
• The locate for the suffix can come before the parent
name or after.
Nomenclature of Alcohols
• For the rules, we will take our basic rules for
alkanes and just adapt them to handle alcohols.
• Rules:
1) Select the longest continuous chain of carbons
which contains the carbon bonded to the
hydroxyl group.
Change the parent name by dropping the –e
and adding –ol
Nomenclature of Alcohols
2) Number the longest continuous chain so as to
give the carbon atoms bearing the –OH the
lowest number
Compounds with 2 –OH groups are named as
diol, but you don’t drop the –e.
Common Names for Alcohol
• Alcohols are named as alkyl alcohol in the
common system.
Naming Monocyclic Compounds
• Monocyclic Compounds- hydrocarbons that
only contain 1 ring.
• Add the prefix cyclo- to the alkane name
having the same number of carbons
Substituted Cycloalkance
• These are named as:
– Alkylcycloalkanes
– Halocycloalkanes
– Alkylcycloalkanols, etc
• If only one substituent is present, no need to number
• When two substituents are present, start numbering at
the substituent first in alphabetical precedence and
continue in the direction to give the next substituent
the lowest possible number
Substituted Cycloalkance
• When three or more are present, we begin at
the substituent that gives the lowest set of
numbers.
• All that being said, group priority still
overrules the above!
Substituted Cycloalkance
• When single ring system is attached to a single
chain with a greater number of carbons or
when there are more than one ring systems,
rings are named as a prefix as cycloalkyls.
• We are not covering the naming of Bicyclic
Compounds!
Naming Alkenes/Cycloalkenes
• Rules:
1) Determine the longest continuous chain that
contains both carbons of the double bond.
Name as the alkane, but drop the –ane and add –ene
2) Number the chain in order to give the carbons
of the double bond the lowest numbers
the locant for the double bond will be the number
of the first carbon of the double bond
Naming Alkenes/Cycloalkenes
3) Indicate substituents groups by the number
of the carbon they are attached to
4) Number substituted cycloalkenes in the way
that gives the carbons in the double bond the 1
and 2 position and that also gives the
substituent groups the lower number at the first
point of difference
Naming Alkenes/Cycloalkenes
5) Name compounds containing both an alcohol
and a double bond as alkenols or cycloalkenols
and give the carbon bonded to the –OH group
the lowest number.
6) Double bonds are sometimes named at
substituents. They are named as alkenyls.
Naming Alkenes/Cycloalkenes
7) Remember that all double bonds that qualify
for cis/trans isomers must be identified by their
configuration. The cis/trans designators go at
the very beginning of the name. In most double
bonds, cis/trans is determined by the
orientation of the parent chain.
Nomenclature of Alkynes
• Same as alkenes, except you drop the –ane
and add –yne.
• Double bonds have priority over triple bonds
Review of Priorities in Nomenclature
• Of the groups covered thus far:
1) Alcohols
2) Double bonds
3) Triple bonds
4) Halo/Alkyl substituents
More on Alkynes
• Monosubstituted Alkynes are called terminal
alkynes and the hydrogen attached is called
an acetylenic hydrogen.
• The anion obtained by removing the
acetylenic hydrogen is named as an alkynide
ion.
Physical Properties of Alkanes and
Cycloalkanes
• Boiling Point
– In general, BP increases as MW increases
– Branching lowers BP
• Density
– As a class, alkanes and cycloalkanes are the least
dense of all organic compounds
– Density is considerably less than water so they float
on top of water
Physical Properties of Alkanes and
Cycloalkanes
• Solubility– Since alkanes and cycloalkanes are essentially
completely nonpolar, they are insoluble in water
– They do dissolve in each other and other nonpolar
solvents.
Sigma Bonds and Bond Rotation
• Conformations- temporary molecular shapes that
result from the rotation of groups about a single
bond
• Each possible structure is called a conformer
• An analysis of the energy changes associated with
the molecule undergoing rotation about a single
bond is called conformational analysis
More structural formulas
• Dash formulas
• 3D dash formulas
• Saw-horse structures
• Newman Projections
Conformational analysis
• The energy difference between conformations
can be shown on a potential energy diagram
• The difference in energies is called the
torsional barrier of a single bond and hinders
free rotation.
Conformational Analysis of Ethane
What does this mean?
• Ethane will spend most of the time in the
lowest energy, staggered conformation, or
close to it.
• Occasionally, it will acquire enough energy to
overcome the torsional barrier and rotate
through the eclipsed conformation.
Definitions
• Resistance to rotation is collectively called torsional
strain.
• One component of torsional strain is Steric
Hindrance.
• Steric Hindrance- An effect on relative reaction
rates caused when the spatial arrangement of
atoms or groups at or near the reacting site hinders
or retards a reaction
Conformational Analysis of Butane
• The two Gauche conformation are stereoisomer
• Since they can be interconverted by the rotation
around a bond, they are called Conformational
Stereoisomers
Relative Stability of Cycloalkanes
• Cycloalkanes do not all have the same relative
stability
• Cyclohexane is the most stable cycloalkane and
cyclopropane and cyclobutane are much less stable
• The relative instability of cyclopropane and
cyclobutane is a direct consequence of their cyclic
structure, and are said to posses ring strain.
Origin of Ring Strain
• The carbon atoms of alkanes and cycloalkanes
are sp3 hybridized so they should have bond
angles of 109.5o
• In cyclopropane, it forms a triangle, so the
bond angles inside the ring must be 60o
• That is 49.5o smaller than the 109.5o
• This compression of the internal bond angle is
called Angle Strain.
Angle Strain
• The sp3 orbitals can not overlap efficiently,
therefore causes the carbon-carbon bonds to
be weaker
• As a result, the molecule has greater potential
energy
• While the angle strain of cyclopropane
accounts for most of the ring strain, it does
not account for all of it.
Origin of Ring Strain
• In addition to angle strain, we have some
torsional strain as well because the hydrogens
are eclipsed.
• So ring strain = angle strain + torsional strain
Cyclobutane
• Cyclobutane also has considerable angle strain
• The internal angles are 88 degrees, a 21o
departure from 109.5o
• The cyclobutane ring is not planar, it is slightly
folded
• If cyclobutane were planar, the angle strain
would be slightly less with 90o bond angles.
Cyclobutane
• However, if it were planar, the torsional strain
would be much greater because all 8 C-H
bonds would be eclipsed
• By folding or bending slightly, the ring
relieves more torsional strain than it gains in
angle strain.
Cyclopentane
• The internal angles of a regular pentagon is 108o,
very close to the 109.5o, of a normal sp3 carbon
• Therefore, if cyclopentane were planar, it would
have very little angle strain, however, it would have
a lot of torsional strain because all 10 C-H bonds
would be eclipsed.
• As a result, cyclopentane assumes a slightly bent
conformation in which one or two of the atoms of
the ring are out of the plane
Cyclopentane
• This relieves some of the torsional strain and
allows for slight twisting of the C-C bonds with
little change in energy
• This allows the out of plane carbons to move
into the plane and another to move out.
• This conformation is called the envelop
conformation.
Conformations of Cyclohexane
• There is considerable evidence that the most stable
conformation of cyclohexane is the Chair
conformation.
• It is non-planar and all carbon-carbon bond angles
are 109.5o, therefore, no angle strain!
• There is also no torsional strain since all bonds are
staggered.
• Plus, the hydrogens atoms at opposite ends are
maximally separated
Conformations of Cyclohexane
• By partial rotation about the C-C bonds, the ring can
assume another conformation called the boat
conformation
• Like the chair, the boat is also free from angle strain
• But the boat is not free of torsional strain, some of
the C-H bonds are eclipsed.
• Additionally, 2 of the H’s are close enough to cause
van der Waals repulsion
Conformations of Cyclohexane
• These repulsions have been called the Flagpole interactions of the boat conformation
• Combined, the torsional strain and the flag
pole interactions cause the boat to have
considerably higher energy than the chair
• Although it is more stable, the chair is more
rigid than the boat
• The boat is quite flexible
Conformations of Cyclohexane
• This flexibility allows the boat to form a new
conformation called the twisted boat.
• The twisted boat relieves some torsional strain
and reduces the flag pole interactions, but is
still 21 kJ/mol more unstable than the chair.
• Because of the stability of the chair, more than
99% of the molecules are in the chair
conformation at any given moment
Positions on a Chair
• Six membered rings are the most common in
nature so we will pay special attention to
them
• In the chair, there are two types of H’s:
• Equatorial hydrogens- lie around the
perimeter of the ring
• Axial hydrogens- lie perpendicular to the ring,
above and below the ring
Positions on a Chair
• The cyclohexane rings flips back and forth
between two equivalent chair conformations
• When the ring flips, all bonds that were axial
become equatorial and vice versa
• If one of the H’s is replaced with a substituent,
the two chair conformations are no longer
equal
Methyl Cyclohexane
• There are two possible chair conformations
• The reason for this is due to the 1,3-diaxial
interactions
• This is created by the steric hindrance when the
methyl is in the axial position
1,3-diaxial interactions
• The strain caused by the 1,3-diaxial interaction is
the same as the strain caused by the close proximity
of the methyl group in the gauche form of butane.
• Recall that the butane-gauche interaction was equal
to 3.8 kJ/mol
• When we look at a newman projection of methyl
cyclohexane with the methyl in the axial position,
we see that there are actually two butane-gauche
interactions
1,3-diaxial interactions
• There are 2 butane-gauche interactions so
that is a total of 7.6 kJ/mol
• As the size of the substituent increases, the
energy difference increases as well
• Example- t-butyl cyclohexane
Disubstituted cycloalkane: Cis/Trans
Isomerism
• The presence of two substituents on the ring of
cycloalkane allow for the possibility of cis/trans
isomerization
• Consider 1,2-dimethylcyclopentane
• The cis/trans versions are stereoisomers, that is,
they differ only in arrangement of their atoms in
space.
Disubstituted cycloalkane: Cis/Trans
Isomerism
• Unlike conformational stereoisomers, they can
not be interconverted without breaking bonds
• As a result, cis/trans isomers can be
separated, placed in separate containers, and
stored indefinitely.
• 1,3-dimethyl cyclopentane show cis/trans
isomerism as well
Disubstituted cycloalkane: Cis/Trans
Isomerism
• Cyclohexane will also show cis/trans with 1,2; 1,3;
and 1,4 substitutions patterns.
• Problem, these flattened rings do not accurately
represent the conformations
• Deciding cis/trans in a chair conformation is
sometime difficult.
Cis/trans in a Chair
• Three ways:
1) Flatten the ring
2) Recognize that there is an upper bond and a
lower bond on each carbon of the chair
3) Conversion Key
Apply what we know!
• Knowing what we know now, we can look at a
name and determine which is the major
conformation
• Ex 1- cis-1,4-dimethylcyclohexane
• Ex 2- trans-1-t-butyl-3-methylcyclohexane
Skipping section 4.14
• Deals with bicyclorings
Chemical Reactions of Alkanes
• Alkanes as a class are usually characterized by the
general inertness to most chemical reagents
• Reasons:
1) C-C and C-H bonds are quite strong and require very
high temperatures to break
2) C and H have nearly the same electronegativity so the
bonds are not polarized
3) No unshared electrons to react
Chemical Reactions of Alkanes
• There are exceptions:
• Alkanes react with Oxygen in combustion
reactions
• They also react with halogens which we will
see in Chapter 10.
Synthesis of Alkanes
1) Hydrogenation of Alkenes and Alkynes
Double bonds and triple bonds can be
reduced to the alkane with H2 gas in the
presence of a metal catalyst such as Pt, Pd, or Ni
Ex.
These reactions are usually done with an alcohol
solvent such as ethanol.
Synthesis of Alkanes
• Not in Book!!
2) Reduction of Alkyl Halides
Most alkyl halides react with Zn and
aqueous acid to produce an alkane
Ex.
Synthesis of Alkanes
• Not in Book!
3) Alkylation of terminal alkynes
We can replace acetylenic H’s with an alkyl
group.
This is called Alkylation and is very useful
in synthesis!
We must first remove the H with a strong
base. NaNH2 is usually used.
Synthesis of Alkanes
• The anion can then be combined with a
methyl halide or a primary alkyl halide
provided that there is no branching at the
beta carbon.
• Ex.
Synthesis of Alkanes
• If the alkyl halide used is a secondary, tertiary,
or a primary with branching at beta carbon,
the reaction gives different products by a
mechanism called elimination, which will be
discussed in chapter 7
Sample Problem
• Synthesize 2-methylpentane from 3-methyl-1butyne and bromomethane.
General Principles of Structure and
reactivity
• The alkylation of the alkynide anion illustrates
several essential aspects of structure and
reactivity
• First, the preparation of the alkynide anion
involves simple Bronsted-Lowry acid-base
chemistry
• Once formed the anion is a lewis base with
which the alkyl halide reacts as an electron
pair acceptor (lewis acid)
General Principles of Structure and
reactivity
• The anion can be called a nucleophile because of
the negative charge concentrated at the terminal
carbon
• Conversely, the alkyl halide can be called an
electrophile because of the partial positive charge
at the carbon bearing the halogen
• In Summary, many of the reactions in organic
chemistry involves acid-base transformations and
interactions of reagents with complementary
charges.
Introduction to Organic Synthesis
• Organic Synthesis is the process of building
organic molecules from simpler precursors.
• Reasons for Organic Synthesis:
1) Create compounds for specific use, ie drugs
2) Create molecules to prove or disprove a
hypothesis. Ie usually labeled with D, T or 13C
• Very simple organic synthesis may only involve
one or two chemical reactions, while others
often require many more
Introduction to Organic Synthesis
• Synthesis of vitamin B12 took 11 years, nearly
100 people and more than 90 step!
• An organic synthesis typically involves two
types of transformations:
1) Reactions that convert functional groups from
one to another
2) Reactions that create new C-C bonds
Introduction to Organic Synthesis
• We have covered examples of both!
• Retrosynthesis- process of starting with the
target molecule, identifying precursors, then
finding how to make precursors, till you get to
readily available starting materials.