Chemistry 11 - Weebly · 2019-09-20 · Chemistry 11 Unit 10 –Organic Chemistry Part II...
Transcript of Chemistry 11 - Weebly · 2019-09-20 · Chemistry 11 Unit 10 –Organic Chemistry Part II...
Chemistry 11Unit 10 – Organic Chemistry
Part II – Alkanes, cycloalkanes and
derivatives
1. Introduction to hydrocarbons
As we have seen in Chapter 8, carbon can form
maximum four bonds with neighboring atoms. In
addition, carbon demonstrates a very unique
characteristic of catenation; carbon atoms can
connect together in arbitrary length.
When a compound is formed by only carbon and
hydrogen, it is called a hydrocarbon. There are a
huge number of types of hydrocarbons, both
aliphatic and aromatic, and they can be found
mainly in crude oil.
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Some of the hydrocarbons extracted from petroleum. 3
Hydrocarbon compounds can be divided into two
classes:
(1) Aromatic: compounds that have flat and ring
structures with resonance bonds. We will discuss it
later in the chapter.
(2) Aliphatic: compounds that are either open-
chain or cyclic, saturated or unsaturated, but do
not contain the aromatic ring. (In other words, non-
aromatic.)
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Depending on the structural features, aliphatic
hydrocarbons can be divided into several families.5
Before we proceed to studying different classes of
hydrocarbons in detail, we need to learn how to
write the structures of hydrocarbons correctly.
There are 3 ways that we can represent the structure
of a hydrocarbon. Namely:
(1) Full structure
(2) Condensed structure
(3) Carbon skeleton
Of course, we can also describe the compound by
means of its molecular formula, but it is not unique.
Many compounds may have the same formula.
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For example: Propane molecule C3H8
Full structure:
Condensed structure:
Carbon skeleton:
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2. Alkanes
(a) Structures
Alkanes form the simplest class of organic
compounds in which all members contain only
single-bonded carbon and hydrogen atoms.
Each carbon atom in an alkane
molecule is bonded to four
neighboring atoms (either
carbon or hydrogen), and is in
tetrahedral geometry. (Recall
the Lewis structures and VSEPR)
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Example: Methane (CH4)
All alkane molecules fulfill the general molecular
formula 𝐂𝒏𝐇𝟐𝒏+𝟐, where 𝑛 is the number of carbon
atoms. Molecules that possess the same general
molecular formula form a homologous series.
Examples:
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C2H6 C4H10 C6H14
C3H8 C5H12 C7H16
In addition to the linear structure, alkanes can also
exist in branched configurations. The structures
having the same molecular formula but different
arrangements are called structural isomers or
constitutional isomers.
For instance, C5H12 has 3 possible structural isomers;
namely
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The number of possible isomers increases dramatically
with the number of carbon atoms. 11
0
1E+09
2E+09
3E+09
4E+09
5E+09
0 10 20 30
Nu
mb
er
of
iso
me
rs
Number of carbon atoms
Number of structural isomers of alkanes
Interestingly, if you re-plot the data in logarithmic
scale, the following graph will be obtained:12
1
10
100
1000
10000
100000
1000000
0 5 10 15 20
log
(Nu
mb
er
of
iso
me
rs)
Number of carbon atoms
Number of structural isomers of alkanes
(b) Nomenclature
(i) For straight-chained alkanes, the naming consists of
two steps:
Step 1: Count the number of carbon atoms and
determine the appropriate prefix:
Step 2: Add the ending –ane to the prefix.
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n prefix n prefix
1 Meth- 6 Hex-
2 Eth- 7 Hept-
3 Prop- 8 Oct-
4 But- 9 Non-
5 Pent- 10 Dec-
Example: Name the following compound.
Example: Draw the structure of a straight chain
nonane.
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(ii) For branched alkanes, the procedures of naming
are little bit more complicated.
Let’s consider the following alkane:
Step 1: Identify the longest straight carbon chain in
the molecule. It is called the parent chain.
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Step 2: Identify all the side chains
attached to the parent chain. Each
side chain, called substituent, is
named by combining the prefix,
which indicates the number of
carbon atoms involved, with the
ending –yl.
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The longest
consecutive
chain contains
9 carbon
atoms
Step 3: Number the parent chain starting either from
the left or from the right and label the locations of
substituents. Choose the scheme that gives the
lowest set of numbers.
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Substituents on carbons
3, 4, 5, 5, 6, 6
It is preferred!
Substituents on carbons
4, 4, 5, 5, 6, 7
It is not preferred!
Step 4: Following the alphabetically order, name the
substituent by the carbon to which it is attached,
followed by a dash, and then the name of the alkyl
group.
e.g. 3-methyl in
Step 5: When there is more than one such a
substituent on the parent chain, name the substituent
by the carbons, in ascending order, followed by a
dash, and then the name of the alkyl group with the
numeric prefix.
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Note that the numeric prefix is different from the
prefix used to designate the carbon chain.
Example: 2,3-dimethyl in
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Number Prefix Number Prefix
1 Mono- 3 Tri-
2 Di- 4 Tetra-
On carbon
#2On carbon
#3
The compound should therefore be named as
follows:
4,5,5-triethyl-3,6,6-trimethylnonane
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Practice: Name the following alkanes.21
(c) Properties
Since C and H have similar values of electronegativity,
C-H and C-C bonds are essentially non-polar, making
alkane molecules non-polar overall.
Due to the lacking of bond dipoles, alkane molecules
are held together by only weak London force. This
results in relatively low melting and boiling points
compared to other organic compounds.
Alkanes are soluble to non-polar or slightly polar
solvents such as benzene, chloroform and ether.
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The following diagram shows the melting and boiling
points of straight chain alkanes.
The higher the number of carbons, the higher the
molecular mass, and the higher the m.p. and b.p.
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Branched alkanes have lower melting and boiling
points than their linear counterparts.
The key: symmetry and packing of molecules
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(d) Cycloalkanes
When C atoms are connected in a ring form, the
resulting alkane is called cycloalkane.
Cycloalkanes are named in a similar way as the linear
alkanes except that the prefix cyclo- is added to the
name of the parent chain.
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Cyclopropane
Cyclobutane
Cyclopentane
Cyclohexane
Because of the ring connectivity, cycloalkanes have
two hydrogen less than the linear counterparts and
share the general formula 𝐂𝐧𝐇𝟐𝐧.
Despite the ring structure, cycloalkanes are not
planar at all, even for the smallest member
cyclopropane. It is in contrast to the aromatic
compounds such as benzene which is fully planar.
(We will discuss it later in this Unit).
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Substituted cycloalkanes are named in the same
way as branched alkanes except for the followings:
(1) No number is needed to indicate the position
of the side chain if there is only one substituent.
(2) If there are two or more substituents, arrange
them in alphabetical order, and the carbon on
which the first substituent is attached is labeled
“1”. The remaining carbons are numbered either
clockwise or counter-clockwise in such a way a
smallest set of numbers is obtained.
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Cycloalkanes can exist as substituents, and to
denote them as side chains, we add the suffix –yl to
the name to the parent cycloalkanes.
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Methylcyclohexane 1-ethyl-3-methylcyclohexane
Substituent Name Substituent Name
C3H5- Cyclopropyl C5H9- Cyclopentyl
C4H7- Cyclobutyl C6H11- Cyclohexyl
(e) Haloalkanes
The hydrogen atoms on an alkane molecule can be
replaced by halogen atoms (i.e., F, Cl, Br or I) to yield
the derivative called haloalkanes or alkyl halides.
To name alkyl halides, we simply consider halogen
groups as substituents and label them with
appropriate positions and prefixes.
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Group Name Group Name
F- Fluoro- Br- Bromo-
Cl- Chloro- I- Iodo-
Practice: Name the following compounds.30
2-fluorobutane 2,4-dichloropentane
2-bromo-3-ethyl-3-fluoropentane
Some properties of alkyl halides
(1) C-X bonds are mostly polar, yet they tend to be
insoluble in water.
(2) Alkyl halides have a various level of reactivity
depending on the type of halogen. Alkyl halides
with fluorine are unreactive (e.g. Teflon) while those
with iodine are quite reactive (e.g. CH3I as
pesticide).
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