Download Branched-Chain Alkanes

Simple Organic Chemistry
Basic Structure and Nomenclature
Graphic: www.lab-initio.com
OBJECTIVES:
– Describe the relationship
between number of valence
electrons and bonding in
carbon.
– Define and describe alkanes.
– Relate the polarity of
hydrocarbons to their
solubility.
Organic Chemistry and
Hydrocarbons
• “Organic” originally
referred to any
chemicals that came
from organisms
• 1828 - German chemist
Friedrich Wohler
synthesized urea in a
lab from inorganic
substances.
1800 – 1882
Organic Chemistry and
Hydrocarbons
• Over a million organic compounds,
with a dazzling array of properties
• Why so many? Carbon’s unique
bonding ability!
• Let’s start with the simplest of the
organic compounds. These are the
Hydrocarbons
• Hydrocarbons contain only two
elements: 1) hydrogen, and 2)
carbon
– simplest hydrocarbons called
“alkanes”, which contain only carbon to
carbon single covalent bonds
• methane (CH4) with one carbon is the
simplest alkane. It is the major
component of natural gas
• Carbon has 4 valence electrons,
thus forms 4 covalent bonds
– not only with other elements, but
also forms bonds WITH ITSELF
(nonpolar)
Representing Hydrocarbons
The table below shows the structural
formulas, ball-and-stick models, and
space-filling models for methane and
ethane.
Different Ways of Representing Hydrocarbons
Name
Methane
Ethane
Structural
formula
Ball-and-stick
model
Space-filling model
Properties of Hydrocarbons
– The electron pair in a carbon-hydrogen or a
carbon-carbon bond is shared almost
equally by the nuclei of the atoms forming
the bond.
– Thus, hydrocarbons are nonpolar molecules.
• The attractions between nonpolar molecules are
weak van der Waals forces.
• So, hydrocarbons with low molar masses tend to
be gases or liquids that boil at a low
temperature.
Properties of Hydrocarbons
Recall the general rule
“like dissolves like.”
– A nonpolar compound
and a polar compound
will not form a
solution.
– For example, because
oil is a mixture of
hydrocarbons, oil and
water do not mix.
Alkanes
– Methane and ethane are examples of
alkanes.
– An alkane is a hydrocarbon in which there
are only single covalent bonds.
– The carbon atoms in an alkane can be
arranged in a straight chain or in a chain
that has branches.
Straight-Chain Alkanes
– Ethane is the simplest straight-chain
alkane, which is an alkane that contains any
number of carbon atoms, one after the
other, in a chain.
• Propane (C3H8) has
three carbon
atoms.
• Butane (C4H10) has
a chain of four
carbons.
Propane
Butane
Straight-Chain Alkanes
200
Temperature (C)
Melting and
boiling points
vary with the
number of
carbons in
straight-chain
alkanes.
Melting and Boiling Points
Of Straight-Chain Alkanes
100
0
–100
–200
0
2
4
6
Number of carbons
8
10
Straight-Chain Alkanes
Homologous series:
The constant increment of change
Ethane Increment
(C2H6) of change
Propane
(C3H8)
Increment
of change
Propane
(C3H8)
Butane
(C4H10)
First Ten Alkanes
Formula Name
Formula Name
CH4
Methane
C6H14
Hexane
C2H6
Ethane
C7H16
Heptane
C3H8
Propane
C8H18
Octane
C4H10
Butane
C9H20
Nonane
C5H12
Pentane
C10H22
Decane
Alkane = CnH2n+2
International Union of Pure and Applied Chemistry (IUPAC).
Straight Chain Alkanes aren’t “Straight”
C – C bonds are sp3 hybridized
CH3
H3C
Butane, C4H10
Alkanes
Formulas for Butane
Formula
C4H10
Description
Molecular formula
Complete structural formula
CH3–CH2–CH2–CH3
CH3CH2CH2CH3
CH3(CH2)2CH3
C–C–C–C
Condensed structural formula
(C–H bonds understood)
Condensed structural formula
(C–H and C–C bonds understood)
Condensed structural formula
(all bonds understood)
Carbon skeleton (hydrogens and C–H
bonds understood)
Line-angle formula
(carbons and hydrogens understood)
Condensed Structural Formula
Explicit hydrogens (those required to complete
carbon’s valence) are usually left off of
drawings of hydrocarbons
H
CH3
H
H
H
H3C
H
C1 C2 C3 C4
H
H
H
H
H
C1 C2 C3
C4
Line intersections represent carbon atoms
Straight-Chain Alkanes
– A condensed structural formula in the form
of CH3(CH2)nCH3 uses a different kind of
shorthand.
• The CH2 unit in parentheses is called a
methylene group.
• The subscript n to the right of the parentheses
indicates the number of methylene groups that
are linked together.
Straight-Chain Alkanes
– This shorthand method applies to butane as
follows:
– The subscript after the parentheses in the
condensed structural formula for butane is
2.
• This means two methylene groups are linked
together in the structure.
Drawing Structural Formulas
for Alkanes
Draw complete structural
formulas for the straightchain alkanes that have
a. three carbon atoms.
b. four carbon atoms.
1
Solve Apply the concepts to this
problem.
Start with the number of carbons.
a.
b.
2
Solve Apply the concepts to this
problem.
Each center carbon bonds to two
hydrogens. Each end carbon bonds to
three hydrogens.
a.
b.
Make sure each carbon has 4 bonds.
Alkanes
Branched-Chain Alkanes
– In organic chemistry, branches on a hydrocarbon
chain are discussed as if they were substituted for
a hydrogen atom on the chain.
• An atom or group of atoms that can take the
place of a hydrogen atom on a parent
hydrocarbon molecule is called a substituent.
Branched-Chain Alkanes
– The longest continuous carbon chain of a
branched-chain hydrocarbon is called the
parent alkane.
– All other carbon atoms or groups of carbon
atoms are regarded as substituents.
Branched-Chain Alkanes
– A hydrocarbon substituent that is
derived from an alkane is called an alkyl
group.
• You can think of any alkyl group as just an
alkane with one of the hydrogens removed.
– Alkyl groups are named by removing the
-ane ending from the parent
hydrocarbon name and adding -yl.
Branched-Chain Alkanes
• The three smallest alkyl groups are
• the methyl group (–CH3)
• the ethyl group (–CH2CH3)
• the propyl group (–CH2CH2CH3)
• An alkane with one or more alkyl
groups is called a branched-chain
alkane.
Branched-Chain Alkanes
– Each carbon in an organic molecule can be
categorized as a primary, secondary, tertiary,
or quaternary carbon.
• If the carbon in question has only one carbon
attached to it, then the carbon is a primary carbon.
• If two carbons are attached to the carbon in
question, the carbon is a secondary carbon; if three
carbons, a tertiary carbon; and if four carbons, a
quaternary carbon.
Branched-Chain Alkanes
Examples of primary, secondary, tertiary,
and quaternary carbons are labeled in the
structural formulas below.
Branched-Chain Alkanes
– Isooctane is a hydrocarbon that contains
each of these types of carbons.
• Isooctane is the standard for determining
octane ratings of the mixtures of hydrocarbons
that make up gasoline.
Branched-Chain Alkanes
– The IUPAC rules for naming branchedchain alkanes are quite straightforward.
• The name of the branched-chain alkane is
based on the name of the longest continuous
carbon chain.
• Each substituent is named according to the
length of its chain and numbered according
to its position on the main chain.
Branched-Chain Alkanes
The compound with the following structural
formula can be used as an example.
Branched-Chain Alkanes
1. Find the longest continuous chain of carbons in
the molecule. This chain is considered the
parent hydrocarbon.
• The longest chain is highlighted in the
example. It contains seven carbon atoms.
So, the parent hydrocarbon is heptane.
Branched-Chain Alkanes
2. Number the carbons in the main chain in
sequence. To do this, start at the end that will
give the substituent groups attached to the chain
the smallest numbers.
• Numbering the chain from right to left gives
the substituents the lowest numbers (2, 3,
and 4).
Branched-Chain Alkanes
3. Add numbers to the names of the substituent
groups to identify their positions on the chain.
These numbers become prefixes to the name of
the substituent group.
• The substituents and positions are 2-methyl,
3-methyl, and 4-ethyl.
Branched-Chain Alkanes
4. Use prefixes to indicate the appearance of the
same group more than once in the structural
formula. Common prefixes are di- (twice), tri(three times), and tetra- (four times).
• The two methyl groups are combined as 2,3dimethyl in the name.
Branched-Chain Alkanes
5. List the names of alkyl substituents in
alphabetical order. For purposes of
alphabetizing, ignore the prefixes di-, tri-, and so
on.
• The 4-ethyl group is listed first, followed by
the 2,3-dimethyl.
Branched-Chain Alkanes
6. Combine all the parts and use proper
punctuation. Write the entire name without any
spaces. Use commas to separate numbers, and
use hyphens to separate numbers and words.
• The correct name of the compound is 4ethyl-2,3-dimethylheptane.
Naming Branched-Chain Alkanes
Name this compound using the
IUPAC system. Notice that the
longest chain is not written in a
straight line.
1
Solve Apply the concepts to this
problem.
Identify the longest
carbon chain in the
molecule.
• The longest chain
has six carbons,
so the name ends
with hexane.
2 Solve Apply the concepts to this problem.
Identify the
substituents and their
positions on the
parent hydrocarbon.
• There are two
methyl
substituents on
carbon 3, so the
Remember to start
prefix is 3,3numbering at the end that
dimethyl.
gives the substituents the
smallest numbers!
2
Solve Apply the concepts to this
problem.
Put everything
together. You can
skip the alphabetizing
step because there is
only one type of
substituent.
• The correct
IUPAC name is
3,3dimethylhexane.
Alkanes
With the name of a branched-chain alkane and knowledge
of IUPAC rules, it is easy to reconstruct the structural
formula.
1. Find the root word (ending in -ane) in the
hydrocarbon name.
2. Then, draw the longest carbon chain to create the
parent hydrocarbon, and number the carbons in the
chain.
3. Identify the substituent groups in the hydrocarbon
name. Attach the substituents to the numbered
parent chain at the proper positions.
4. Complete the structural formula by adding
hydrogens as needed.
Drawing Structural Formulas
for Branched-Chain Alkanes
Draw the structural formula
for 2,2,4-trimethylpentane,
or isooctane.
2 Solve Apply the concepts to this
problem.
Draw the structural formula for 2,2,4trimethylpentane.
Draw the number of carbons needed to
represent the parent structure indicated in
the name, and number the carbons on the
chain.
•
The parent structure is pentane, which
has five carbon atoms.
2 Solve Apply the concepts to this
problem.
Draw the structural formula for 2,2,4trimethylpentane.
Attach each substituent as indicated in the
prefix.
•
There are two methyl groups on carbon
2 and one on carbon 4.
2 Solve Apply the concepts to this
problem.
Draw the structural formula for 2,2,4trimethylpentane.
Finish by adding hydrogens where needed in
the formula.
•
A total of nine hydrogens need to be
added to complete the structure.
Be careful:
Each carbon has
four, and only
four, bonds.
Nomenclature Practice
Name this compound
CH3
H3C1 2
Cl
3 4
5
CH3
6
7
9 carbons = nonane
8
H3C9
Step #1: For a branched hydrocarbon, the longest
continuous chain of carbon atoms gives the root name
for the hydrocarbon
Nomenclature Practice
Name this compound
CH3
H3C1 2
Cl
3 4
5
CH3
6
7
8
9 carbons = nonane
CH3 = methyl
chlorine = chloro
H3C9
Step #2: When alkane groups appear as substituents, they
are named by dropping the -ane and adding -yl.
Nomenclature Practice
Name this compound
CH3
H3C1 2
Cl
3 4
5
6
7
9 carbons = nonane
CH3
CH3 = methyl
chlorine = chloro
8
H3C9
1
9 NOT
9
1
Step #3: The positions of substituent groups are
specified by numbering the longest chain of carbon
atoms sequentially, starting at the end closest to the
branching.
Nomenclature Practice
Name this compound
CH3
H3C1 2
Cl
3 4
5
CH3
6
7
9 carbons = nonane
CH3 = methyl
8
chlorine = chloro
H3C9
2-chloro-3,6-dimethylnonane
Step #4: The location and name of each substituent are
followed by the root alkane name. The substituents are
listed in alphabetical order (irrespective of any prefix),
and the prefixes di-, tri-, etc. are used to indicate
multiple identical substituents.
Organic Homework
• Go to the hand out section and download
the page called Organic Chem Homework.
• Please e-mail the completed work to me
by 9:30 pm tonight!!
Take this quiz
Quiz on Naming Alkane
Alkenes Contain Carbon-Carbon
Double bonds
1  bond
H
H
C
H
H
C
H
C
H
H
C
H
Ethene
1  bond
Alkynes Contain Carbon-Carbon
Triple Bonds
H
H
C
C
C
1  bond
C
Ethyne
1  bond
H
1  bond
H
Structural Isomers
Look below at the structures of butane
and 2-methylpropane.
Butane
2-methylpropane
Structural Isomers
– Even though both compounds have the formula C4H10,
their boiling points and other properties differ.
– Because their structures are different, they are
different substances.
– Compounds that have the same molecular formula but
different molecular structures are called isomers.
Butane
2-methylpropane
Structural Isomers
– Butane and 2-methylpropane represent a
category of isomers called constitutional
isomers, or structural isomers.
– Structural isomers are compounds that
have the same molecular formula but the
atoms are joined together differently.
Structural Isomers
Stuructrual isomers differ in physical
properties such as boiling point and
melting point. They also have different
chemical reactivities.
• In general, the more highly branched the
hydrocarbon structure is, the lower the
boiling point of the isomer will be compared
with less branched isomers.
Which of these molecules will have a lower
boiling point and why?
Butane
2-methylpropane
Which of these molecules will have a lower
boiling point and why?
Butane
2-methylpropane
2-methylpropane will have a lower boiling
point because it has a more highly
branched structure.
Stereoisomers
– Because molecules are three-dimensional
structures, molecules with the same
molecular formula and with atoms joined
in exactly the same order may still be
isomers.
– Stereoisomers are molecules in which
the atoms are joined in the same order
but the positions of the atoms in space
are different.
Geometric Isoisomers
– A double bond between two carbon
atoms prevents other atoms in the
molecule from rotating, or spinning, with
respect to each other.
– Because of this lack of rotation, groups
on either side of the double bond can
have different orientations in space.
Geometric Isoisomers
Cis-Trans Isomers
– also known as geometric isomers, have
atoms joined in the same order but the
spatial orientation of the groups
differs.
– The most common example of cis-trans
isomerism occurs in molecules with
double bonds.
Cis-Trans Isomers
Two arrangements are possible for the
methyl groups and hydrogen atoms with
respect to the rigid double bond in 2butene.
Opposite sides
Same side
Cis configuration
Trans configuration
Stereoisomers
Cis-Trans Isomers
•
Cis-trans isomers have
different physical and
chemical properties.
•
You should be able to
identify cis-trans
isomers of alkenes
when each carbon of
the double bond has one
substituent and one
hydrogen.
Stereoisomers
Enantiomers
– The second category of
stereoisomerism occurs whenever a
central atom has four different atoms
or groups attached.
– Most commonly the central atom is
carbon.
Stereoisomers
Enantiomers
•
•
A carbon with four
different atoms or
groups attached is an
asymmetric carbon.
In the compound shown
here, H, F, Cl, and Br
atoms are attached to a
single carbon atom, so
the carbon is an
asymmetric carbon.
CHFClBr
Stereoisomers
Enantiomers
•
The relationship between
the two molecules is
similar to the
relationship between
right and left hands.
•
Sometimes the terms
right-handed and lefthanded are used to
describe compounds with
an asymmetric carbon.
Stereoisomers
Enantiomers
– To understand the stereoisomerism that
involves asymmetric carbons, you need
to visualize the relationship between an
object and its mirror image.
– If the object is symmetrical, like a ball,
then its mirror image can be
superimposed.
•
That is, the appearance of the ball and
its reflection are indistinguishable.
Stereoisomers
Enantiomers
– By contrast, a pair of hands is distinguishable
even though the hands have identical parts.
– The right hand reflects as a left hand, and the
left hand reflects as a right hand.
– When you try to stack your hands on top of one
another, the thumb of one hand lines up with
the little finger of the other hand.
•
No matter how you turn your hands, you
can’t get them to look exactly alike.
Stereoisomers
Enantiomers
– Pairs of molecules that are mirror
images and not superimposable are
called enantiomers, or optical isomers.
•
These molecules are examples of
enantiomers.
Stereoisomers
Enantiomers
•
Unlike other isomers, enantiomers have
identical physical properties such as
boiling points and densities.
•
Enantiomers do, however, behave
differently when they interact with other
molecules that have asymmetric carbons.
– Many molecules in your body have
asymmetric carbons, so each enantiomer
can have a different effect on the body.
Does a compound have to be smelly in
order to be classified as aromatic?
When you hear
the word
aromatic, you may
think of perfume
or flowers.
A compound that contains a hydrocarbon
ring is called a cyclic hydrocarbon.
• Many
molecules
found in
nature contain
cyclic
hydrocarbons.
• Rings with five
and six
carbons are
the most
abundant.
Cyclic Alkanes
Cyclopropane, C3H6
Cyclobutane, C4H8
Cyclopentane, C5H10
Cyclohexane, C6H12
Cycloheptane, C7H14
Remember, explicit hydrogens are left out
Cyclic Hydrocarbons
– Just as straight-chain and branchedchain alkanes can be either saturated or
unsaturated, so can cyclic hydrocarbons.
– A cyclic hydrocarbon that contains only
single bonds, and is therefore saturated
is called a cycloalkane.
Cyclic
Hydrocarbons
Name:
propane
• To determine the
IUPAC name of a
cycloalkane, first
count the number
of carbons in the
ring and assign the
corresponding
alkane name.
Cyclic
Hydrocarbons
• To determine the IUPAC
name of a cycloalkane,
first count the number
of carbons in the ring
and assign the
corresponding alkane
name.
Name: cyclopropane
• Then simply add the
prefix cyclo- to the
alkane name.
How many hydrogen atoms will a
cycloalkane contain in comparison
to a straight chain alkane with the
same number of carbons?
How many hydrogen atoms will a
cycloalkane contain in comparison
to a straight chain alkane with the
same number of carbons?
A cycloalkane will contain two fewer
hydrogen atoms than a straight chain
alkane with the same number of carbons
because there is one additional carboncarbon bond in the cycloalkane.
Aromatic Hydrocarbons
– There is a class of unsaturated cyclic
hydrocarbons that are responsible for
the aromas of spices such as vanilla,
cinnamon, cloves, and ginger.
– These compounds were originally called
aromatic compounds because they have
distinct, pleasant aromas.
– However not all compounds classified
has aromatic have pleasant odors, or any
odor at all.
Aromatic Hydrocarbons
•
Benzene is the
simplest aromatic
compound.
•
An aromatic
compound, or arene, is
now defined as an
organic compound
that contains a
benzene ring or other
ring in which the
bonding is like that of
benzene.
Aromatic Hydrocarbons
The Structure of Benzene
•
The benzene molecule is a six-membered carbon ring
with one hydrogen atom attached to each carbon.
•
This arrangement leaves one electron from each
carbon free to participate in a double bond.
– Two different structures with alternating double
bonds can be written for benzene.
Aromatic Hydrocarbons
The Structure of Benzene
– Drawing a solid or dashed circle inside a
hexagon is a good way to represent
benzene in terms of how the electrons
are distributed.
– However, such a drawing does not show
the number of electrons involved.
Aromatic Hydrocarbons
Cyclic unsaturated hydrocarbons with
delocalized electrons
The simplest aromatic hydrocarbon is
benzene (C6H6)
H
H
H
H
H
H
H
H
H
H
H
OR…
H
H
H
H
H
H
H
Aromatic Hydrocarbons
Substituted Aromatic Rings
– Many dyes used to produce
the intense colors of your
clothing, such as the blue
shown here are
substituted aromatic
compounds.
Aromatic Hydrocarbons
Substituted Aromatic Rings
– Compounds containing substituents
attached to a benzene ring are named
using benzene as the parent
hydrocarbon.
Aromatic Hydrocarbons
Substituted Aromatic Rings
When the benzene ring is a
substituent, the C6H5 group is called
a phenyl group.
Aromatic Hydrocarbons
Substituted Aromatic Rings
– Derivatives of benzene with two
substituents are called disubstituted
benzenes.
– Dimethylbenzene, also called xylene, is
an example of a disubstituted benzene.
Aromatic Hydrocarbons
Substituted Aromatic Rings
– There are three constitutional isomers for
dimthylbenzene (C6H4(CH3)2).
– The boiling points of the three compounds are
a reminder that constitutional isomers have
different physical properties.
Geometric Isomerism in Aromatics
ortho (o-) = two adjacent
substituents
o-dichlorobenzene
Cl
H
Cl
H
H
H
meta (m-) = one carbon between
substituents
m-dichlorobenzene
para (p-) = two carbons between
substituents
p-dichlorobenzene
Cl
H
H
Cl
H
H
Cl
H
H
H
H
Cl
Aromatic Hydrocarbons
Substituted Aromatic Rings
– In the IUPAC naming system, the possible positions
of two substituents in disubstituted benzene are
designated as 1,2; 1,3; or 1,4.
– Common names for disubstituted benzenes use the
terms ortho, meta, and para (abbreviated as o, m,
and p) in place of numbers.
Reactions of Alkenes and Alkynes
Hydrogenation
CH 2  CHCH 3  H 2 Catalyst
 CH 3CH 2 CH 3
Propene
Propane
Halogenation
CH 2  CHCH 2 CH 2 CH 3  Br2  CH 2 BrCHBrCH 2 CH 2 CH 3
1-Pentene
1-2-dibromopentene
Polymerization
Small molecules are joined together to form
a large molecule
CH2 CH2
Polyethylene
n
Hydrocarbon Derivatives
Class
Alcohol
Functional Group
hydroxyl group
General Formula
-O — H
R – OH
Alkyl halide
—X
R—X
Ether
—O—
R — O — R’
Aldehyde
carbonyl group
O
||
—C—H
O
||
R—C—H
Ketone
carbonyl group
O
||
—C—
O
||
R — C — R’
Carboxylic acid
carboxyl group
O
||
— C — OH
Ester
Amine
amine group
O
||
— C — OH
O
||
— C — O—
O
||
R — C — O — R’
|
—N—
R’
|
R — N — R’’