Transcript 3. Organic Compounds: Alkanes and Cycloalkanes

ORGANIC CHEMISTRY:
The Chemistry of Life
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Organic Chemistry
• Organic chemistry is the chemistry
of carbon compounds.
• Carbon has the ability to form
long chains.
• Without this property, large
biomolecules such as proteins,
lipids, carbohydrates, and nucleic
acids could not form.
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Structure of Carbon Compounds
• There are three geometries or hybridization
states or found on each carbon in any organic
compounds:
– Sp3 hybridized - tetrahedral
– sp2 hybridized - trigonal planar
– sp hybridized - linear
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Hydrocarbons
• There are four basic
types of hydrocarbons:
–
–
–
–
Alkanes
Alkenes
Alkynes
Aromatic hydrocarbons
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CHAPTER 2
Organic Compounds: Alkanes
and Cycloalkanes
Alkanes
• Alkanes contain only single bonds.
• They are also known as saturated hydrocarbons.
– They are “saturated” with hydrogens.
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Formulas
• Lewis structures of alkanes look like this
• They are also called complete structural
formulas.
• They are often not convenient, though…,
Complete Structural Formula
Condensed Structural Formulas
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Formulas
…so more often condensed formulas are
used.
Because some organic molecules can consist of thousands of atom an
even more abbreviated Skeletal
Formula was devised.
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Drawing Chemical Structures
• Chemists use shorthand ways for writing structures
• Condensed structures: C-H and C-C single bonds aren't shown
but understood
– If C has 3 H’s bonded to it, write CH3
– If C has 2 H’s bonded to it, write CH2; and so on. The compound called
2-methylbutane, for example, is written as follows:
• Horizontal bonds between carbons aren't shown in condensed
structures—the CH3, CH2, and CH units are simply but vertical
bonds are added for clarity
Complete
Structural
Formula
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Skeletal Structures
• C’s are not shown. They are assumed to be
at each intersection of any two lines (bonds)
and at end of each line
• H’s bonded to C’s aren't shown –Since
carbon always has a valence of 4, we
mentally supply the correct number of H’s by
subtracting the # of bonds shown from 4.
• All atoms other than C and H are shown
• See next slide for examples
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0
H
0
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Multiple Bonds
In condensed formulas double and triple bonds are drawn as they
would be in a Lewis structure showing two dashes for a double
bond and three dashes for a triple bond
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Families of Organic Compounds
• Organic compounds can be grouped into
families by their most important structural
feature – by their:
Functional
Groups
The term functional
group is used to
refer to parts of
organic molecules
where reactions
tend to occur.
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Functional Groups
• A Functional Group is an atom or collection of
atoms that imparts characteristic chemistry to
whatever hydrocarbon skeleton that it is bonded
to.
• The functional group reacts in the same way,
independent of the rest of the molecule
• For example, the double bonds in simple and
complex alkenes react with bromine in the same
way.
Common Rxns of the Double Bond
Functional Group
Survey of Functional Groups
• The inside cover of your text lists the
organic functional groups. You must
memorize them.
• As you learn about them in each chapter it
will be easier to recognize them
• The functional groups affect the reactions,
structure, and physical properties of every
compound in which they occur
For Ease of Study the Functional
Groups can be grouped into a few
types based upon their common
structural features
Types of Functional Groups:
Multiple Carbon–Carbon Bonds
• Alkenes have a CC double bond
• Alkynes have a CC triple bond
• Arenes or
Aromatics have
special bonds that
are represented
as alternating
single and double
C-C bonds in a
six-membered
ring
Functional Groups with Carbon Singly
Bonded to an Electronegative Atom
• Alkyl halide: C bonded to halogen (C-X)
• Alcohol: C bonded O of a hydroxyl group (C
OH)
• Ether: Two C’s bonded to the same O (C O C)
• Amine: C bonded to N (C N)
• Thiol: C bonded to SH group (C SH)
• Sulfide: Two C’s bonded to same S (C S C)
• In all of these functional groups the bond
between Carbon and the electronegative atom is
polar, with partial positive charge on C (+) and
partial negative charge () on electronegative
atom
•
•
•
•
•
•
•
Groups with a Carbon–Oxygen Double
Bond (Carbonyl Groups)
Aldehyde: one hydrogen bonded to C=O
Ketone: two C’s bonded to the C=O
Carboxylic acid: OH bonded to the C=O
Ester: O-C bonded to the C=O
Amide: N-C and/or N-H bonded to the C=O
Acid chloride: Cl bonded to the C=O
In all of these functional groups the Carbonyl C
has a partial positive charge (+) and the
Carbonyl O has partial negative charge (-).
Alcohols
• Alcohols contain one or more
hydroxyl groups, —OH.
• They are named
from the parent
hydrocarbon; the
suffix is changed to
-ol and a number
designates the carbon
to which the hydroxyl
is attached.
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Alcohols
• Alcohols are much more acidic than hydrocarbons.
– pKa ~15 for most alcohols.
– Aromatic alcohols have pKa ~10.
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Ethers
• Ethers tend to be quite unreactive.
• Therefore, they are good polar solvents.
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Aldehydes
In an aldehyde, at least one hydrogen is
attached to the carbonyl carbon.
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Ketones
In ketones, there are two carbons bonded to
the carbonyl carbon.
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Carboxylic Acids
• Acids have a hydroxyl group bonded to
the carbonyl group.
• They are tart tasting.
• Carboxylic acids are weak acids.
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Esters
• Esters are the products of reactions between
carboxylic acids and alcohols.
• They are found in many fruits and perfumes.
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Amides
Amides are formed by the reaction of
carboxylic acids with amines.
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Amines
• Amines are organic bases.
• They generally have strong, unpleasant odors.
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Circle and name the functional group(s) in
the following molecules.
amide
amine
alkene
ester
arene
Carboxylic
acid
alkene
alkene
ketone
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alcohol
Circle and name the functional groups
alcohol
amine
arene
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Circle and name the functional groups
Alkane
arene
Carboxylic
acid
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The Functional Group that we will focus
on in this chapter is the Alkane
• Alkanes: Compounds with C-C single bonds and C-H
bonds only
• The general formula for an alkane with no rings in it
must be CnH2n+2 where the number of C’s is n
• Alkanes are said to be saturated with hydrogens (no
more can be added)
• They are also called aliphatic compounds or parrafins
n-
This is a Saturated Animal Fata Triglyceride
The Names for the first 10 Alkanes must be
committed to memory
No. of
Carbons
1
2
3
4
5
6
7
8
9
10
Formula Name
(CnH2n+2)
Methane
Ethane
Propane
Butane
Pentane
Hexane
Heptane
Octane
Nonane
Decane
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C7H16
C8H18
C9H20
C10H22
Alkane Isomers
• When we get to Butane, C4H10, we find
that there are two different structures that
can share the same formula;
– These are: n-Butane and Isobutane or 2methylpropane. They are isomers of one
another
CH3CH2CH2CH3
n-
(CH3)3CH
There are different kinds of Isomers
• Isomers that differ in how their atoms are arranged
in chains are called constitutional isomers
• Compounds other than alkanes can be
constitutional isomers of one another
• Constitutional isomers can be skeletal, functional
group or positional
. If we continue to build succeedingly
larger alkanes according to the
general formula CnH 2n+2, we will
discover that the number of isomers
for each one increases greatly.
The Need for a System of
Nomenclature
• It is obvious that the existence of such vast
numbers of isomers (75 for Decane) require a
system of nomenclature that can identify any
one of these.
• Before enumerating the rules for nomenclature,
there are two items that we must cover.
– Classification of Carbon atoms
– Classification of Hydrogen atoms
Classification of Carbon Atoms
• A carbon atom may be classified by identifying
the number of other carbon atoms that it is
bonded to. Note R is used here and
throughout the notes on organic chemistry to
represent a general hydrocarbon group.
We often use these terms when
speaking, therefore, their meanings
must become second nature.
For example;
Classification of Hydrogen Atoms
• Hydrogens are classified as primary (1°),
secondary (2°) or tertiary (3°) depending
upon the class of carbon that they are
bonded to. How many primary, secondary
and tertiary hydrogens are in the following;
CH3CH(CH3)CH2CH(CH3)2
12, 1°’s
2, 2°’s
2, 3°’s
Classification of Hydrogens
Naming Alkanes
Alkanes are named by a process that uses
Prefix-Parent-Suffix
branching groups
are in longest chain?
2,3-dimethylpentane
Naming Alkyl Groups
These alkyl groups will later appear as branches that
hang off the longest carbon chain of larger organic
molecules. Their names must be committed to memory.
Rules for Naming Alkanes
• Identify the longest continuous carbon
chain = parent name
• Number the carbons of the longest chain
starting from the end that gives the
smallest number for the first branch point.
• Name the molecule by identifying the
name of each branch and its position on
the longest carbon chain, followed by the
name of the parent.
Find the longest continuous carbon
chain and determine the name of the
parent name
3-methylhexane
4-ethyl-3-methylheptane
Number the Carbon Atoms in the
longest carbon chain- start at the end
that gives the smallest number for the
first branch
Number the carbons in the longest
carbon chain
3-ethyl-4,7-dimethylnonane
Identify and Number the Substituents
4-ethyl-2,4-dimethylhexane
Write the name as a single word
Alternative Method for Naming
Complex Branches
• When naming the more complex branches
(branches that have their own branches),
the branch carbon that is directly attached
to the main chain is labelled as the #1
branch carbon and the branch is then
named as if it were a compound itself.
However the complex branch name still
ends in the suffix –yl. When named this
way, the complex branch name is always
placed within parenthesis. Consider the
following complex branches.
(1-methylpropyl)
( 2-methylpropyl)
( 1,1-dimethylethyl)
Properties of Alkanes
• Alkanes are called paraffins (from the
Greek “para affinis” meaning low affinity
because they have a very low reactivity
• They will burn in a flame, producing
carbon dioxide, water, and heat and…
• They react with Cl2 in the presence of light
to replace H’s with Cl’s (not controlled)
Physical Properties
• Boiling points and melting points increase as size of
alkane increases
• Forces between molecules (intermolecular forces are
London Dispersion forces. These are weak but
increase with increasing molecular weight
Properties of Alkanes
• The only van der Waals force is the
London dispersion force.
• The boiling point increases with the length
of the chain.
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Cycloalkanes: Cis-Trans Isomerism
• Cycloalkanes or alicyclic compounds (aliphatic cyclic) are
rings of carbon atoms that have the general formula CnH2n
• In many respects, the chemistry of cyclalkanes mimic that of
the noncyclic alkanes. They are all nonpolar and chemically
inert to most reagents.
• The main difference between cycloalkanes and noncyclic
alkanes is the lack of complete rotation about the carbon to
carbon ring bond. This decrease in freedom of rotation
about the carbon to carbon bond is most obvious in the
small ring cycloalkanes
• For example, cyclopropane cannot have any carbon to
carbon bond rotation without breaking the ring.
Cyclopropane must be a flat, planar molecule with a rigid
structure
Cis-Trans Isomerism in
Cycloalkanes
• Rotation about C-C bonds in cycloalkanes is limited
by the ring structure
• Rings have two “faces” and substituents are labeled
as to their relative facial positions
• There are two different 1,2-dimethyl-cyclopropane
isomers, one with the two methyls on the same side
(cis) of the ring and one with the methyls on
opposite sides (trans)
Cycloalkanes
• Some two dimensional representations of
cycloalkanes
cyclobutane
cyclopropane
cyclopentane
cyclohexane
Complex Cycloalkanes
• Naturally occurring materials contain cycloalkane
structures
• Examples: chrysanthemic acid (cyclopropane),
prostaglandins (cyclopentane), steroids (cyclohexanes
and cyclopentane)
Naming Cycloalkanes
• Count the number of carbon atoms in the ring and the
number in the largest substituent chain. If the number of
carbon atoms in the ring is equal to or greater than the
number in the substituent, the compound is named as an
alkyl-substituted cycloalkane
• For an alkyl- or halo-substituted cycloalkane, start at a
branch and call its ring carbon, C1 ,and number the
substituents on the ring so that the second substituent
has as low a number as possible.
• Number the substituents and write the name
• See text for more details and examples
Stereoisomers
• Compounds with atoms connected in the same order but
which differ in three-dimensional orientation, are
stereoisomers
• The terms “cis” and “trans” should be used to specify
stereoisomeric ring structures
• Recall that constitutional isomers have atoms
connected in different order
The Shapes of Molecules
• I. Stereochemistry: the branch of chemistry
concerned with the three dimension shapes
of molecules
The most stable shape of Decane.
• A molecule may assume different shapes, called
conformations, that result from rotation about a
carbon-carbon single bond.
• Although there are many possible conformations
available to a particular molecule; each molecule will
spend most of its time in its most stable
conformation.
• The most stable conformation for any molecule is the
one that minimizes the mutual repulsion of bonded
electron clouds on adjacent carbons.
Representing Conformations
• Sawhorse
representations: these
view the C-C bond from
an oblique angle and
indicate spatial
orientations by showing
all the C-H bonds.
• Newman projections:
these site along a
particular C-C bond and
represent the two carbon
atoms by a single circle.
Substituents on the front
carbon are represented
by lines going to the
center of the circle, and
substituents on the rear
carbon are indicated by
lines going to the edge of
the circle.
Conformations of Ethane – Torsional Strain Energy
• Rotation about the C-C bond in ethane is not exactly free.
There is a slight energy barrier (12 kJ/mol) to this rotation.
This barrier stems from the fact that certain conformers
have a higher energy content (are less stable) than others.
The highest energy, least stable conformer is the eclipsed
conformer. In this conformer all six C-H bonds are as
close as possible. Since the energy barrier is 12 kJ/mol
and we can see from the highest energy eclipsed
conformer that this is due to three H,H eclipsing
interactions, then we can assign a value of 4 kJ/ mol for
each H,H eclipsing interaction.
This increased energy due to eclipsing
interactions is called torsional strain and is
one kind of strain energy. Torsional strain is
due to mutual repulsion between electron
clouds as they pass by each other in eclipsed
conformers
Staggered Conformation of
Ethane
• The lowest energy,
most stable
conformer is the
staggered
conformer.
In this conformer, all six CH bonds are as far apart
as possible
Ethane’s Conformations
• There barrier to rotation between conformations is
small (12 kJ/mol; 2.9 kcal/mol) The most stable
conformation of ethane has all six C–H bonds away
from each other (staggered)
• The least stable conformation has all six C–H bonds
as close as possible (eclipsed) in a Newman
projection – energy due to torsional strain
Conformations of Propane – Steric Strain Energy
• The barrier to free rotation about
carbons #2 and #3 is 14 kJ/mol.
Inspection of the highest energy
eclipsed conformer indicate that
this strain is due to two H,H
interactions and one H,C
interaction. We have seen in
ethane that each H,H interaction
accounts for approximately 4
kJ/mole of strain energy. This
means that the C,H interaction
must account for 6kJ/mole of strain
energy. The C,H strain energy of
propane is due to torsional and
steric strain. Steric strain results
from two atoms or groups
attempting to occupy the same
space at the same time.
4.3 Conformations of Butane
• The barrier to free rotation about C2-C3 in butane is 19kJ/mole.
Inspection of the highest energy eclipsed conformation indicates that the
torsional strain is due to two H,H interactions and one C,C interaction.
• This allows us to calculate a value of 11 kJ/ mole for the methyl, methyl
eclipsing interaction.
•Consider the plot of P.E. vs rotation about C2 - C3 in butane and notice that
there are two different staggered conformers that do not have the same energy
and two different eclipsed conformers that do not have the same energy. We
know that the highest energy eclipsed conformer has a total strain of 19
kJ/mole.
0 kJ/mole
3.8 kJ/mole
19 kJ/mole
The Gauche Butane Conformer
•The strain energy of this
conformer results from the fact
conformer is 3.8
that the H’s of both CH3 groups
kJ/mol higher in
are attempting to occupy the
energy than the most same space at the same time.
stable,anti conformer This type of strain is called…
• The energy of this
even though it has no
eclipsing interactions.
Stability of Cycloalkanes: The
Baeyer Strain Theory
• In 1885, Baeyer proposed a theory to explain
the apparent lack of cyclic alkanes having
certain ring sizes. More specifically only 5 and 6
membered cycloalkane rings were known but
smaller and larger rings could not be prepared.
Baeyer theorized that these could not be
prepared because their bond angles would
necessarily deviate from the preferred sp3 bond
angle of 109.5 degrees. This deviation would
cause such angle strain that the rings would be
too unstable to exist.
Bayer’s Proposed Bond Angle Strain
Compound Structure
Baeyer Bond Angle
Angle
Strain
Cyclopropane
60 degrees
Cyclobutane
90 degrees 109-90=19
Cyclopentane
108
degrees
120
degrees
Cyclohexane
109-60=49
109-108=1
109-120= -11
Baeyer concluded that cyclopropane would be the most strained followed by
cyclobutane. Cyclopentane would be strain free while cyclohexane would show
a substantial amount of strain energy. Rings larger than cyclohexane would be
impossibly strained and not capable of existing.
Measurements of Energy
Strain in a Cyclic Compound
•Heats of combustion can be used to measure the total
amount of energy strain in a compound. The notion here is
that the more strained a compound, the higher is its energy
content and the more heat delivered per CH2 unit upon
combustion to CO2 + H2O.
Alkane + O2
CO2 + H2O + Heat
Bayer’s Theory Busted
Heats of combustion data indicate that Baeyer's theory was
not fully correct. Cyclopropane and cyclobutane are quite
strained but cyclopentane is more strained then first
predicted while cyclohexane is less strained. For larger
rings, there is no regular increase in strain and rings of
more than 14 carbons are strain free.
Why was Baeyer's theory incorrect?
Baeyer assumed that all cycloalkanes are flat when in fact
most adopt a puckered 3-D conformation. Furthermore, he
did not consider the contribution of torsional strain to the
overall strain energy of a molecule.
The Nature of Ring Strain
• Rings larger than 3 atoms are not flat
• Cyclic molecules can assume nonplanar
conformations to minimize angle strain
and torsional strain by ring-puckering
Summary: Types of Strain
• Angle strain - expansion or compression
of bond angles away from the preferred
109.5
• Torsional strain - eclipsing of bonds on
neighboring atoms
• Steric strain - repulsive interactions when
two atoms or groups bump in to one
another
Conformations of some
Cycloalkanes
• 3-membered ring must have planar structure
• Symmetrical with C–C–C bond angles of 60°
• Requires that sp3 based bonds are bent (and
weakened)
• All C-H bonds are eclipsed
Conformations of Cyclobutane and
Cyclopentane
• Flat cyclobutane has less angle strain than
cyclopropane but much more torsional strain
because of its larger number of ring hydrogens
• Consequently, cyclobutane is slightly bent out of
plane - one carbon atom is about 25° above
– The bend increases angle strain but decreases
torsional strain
Cyclopentane
• Planar cyclopentane would have no angle strain
but very high torsional strain
• Actual conformations of cyclopentane are
nonplanar because this reduces the torsional
strain somewhat.
• Four carbon atoms are in a plane
– The fifth carbon atom is above or below the plane –
looks like an envelope
Conformations of Cyclohexane
• Cyclohexane compounds are the most important
of all cycloalkanes because of their wide
occurrence in nature. Cyclohexane is a strain
free cycloalkane because of a 3-D conformation
that relieves all strain. The C,C bond angles of
this chair confirmation are all ~ 109 degrees
and the conformation displays no H,H eclipsing
strain
How to Draw Cyclohexane
Axial and Equatorial Bonds in
Cyclohexane
• The chair conformation
has two kinds of
positions for
substituents on the
ring carbons: axial
positions and
equatorial positions
• Chair cyclohexane has
six axial hydrogens
perpendicular to the
ring (parallel to the ring
axis) and six
equatorial hydrogens
near the plane of the
ring
Axial and Equatorial Positions
• Each carbon atom in cyclohexane has one
axial and one equatorial hydrogen
• Each face of the ring has three axial and
three equatorial hydrogens in an
alternating arrangement
Drawing the Axial and Equatorial
Hydrogens
Conformational Mobility of
Cyclohexane
• Chair conformations readily interconvert,
resulting in the exchange of axial and
equatorial positions by a ring-flip
Bromocyclohexane
• When bromocyclohexane ring-flips the
bromine’s position goes from equatorial to
axial and so on
Conformations of Monosubstituted
Cyclohexanes
• The two conformers of a monosubstituted
cyclohexane are not equal in energy
• The equatorial conformer of methyl cyclohexane
is more stable than the axial by 7.6 kJ/mol
Conformational Analysis of
Disubstituted Cyclohexanes
• The most stable conformer is the one that
has the largest group in the equatorial
position.
Conformational Analysis of
Disubstituted Cyclohexanes
• In disubstituted clohexanes the steric effects of
both sbstituents must be taken into account in
both conformations
• Be careful here. Remember that there are four
conformational isomers of 1,2 dimethyl
cyclohexane. There are two cis cinformers
related by a ring flip and two trans related by a
ring flip. Your job is to identify the one with the
least strain energy. The most stable one
• Also remember that each face has alternating
axial and equatorial positions
e
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top
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a
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bottom
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