Saturated hydrocarbons

1
Saturated hydrocarbons

• Chapter 12

2
Organic and inorganic compounds

• Organic chemistry concerns the chemistry of
  carbon compounds ? contain C and H, but also
  maybe other p-block elements.
• At the time of printing (Stoker, 5th edition),
  there were around 5 million organic compounds
  catalogued
• Inorganic chemistry concerns the chemistry of the
  other 117 elements. Around 1.5 million of these
  are known

Non-polar hydrocarbon tail
A polar, charged group
3
Organic and inorganic compounds

• The reason why there are so many organic
  compounds is that carbon is very good at forming
  bonds with other carbon atoms.
• Carbon atoms are commonly found in chain-like
  arrangements or C-rings (or both within the same
  molecule).
• Carbon has four valence electrons. In organic
  compounds, it forms four covalent bonds to obtain
  an octet.

4
Hydrocarbons and hydrocarbon derivatives

• Hydrocarbons are compounds that contain only
  carbon and hydrogen in their formulas.
• Two basic categories of hydrocarbon
• Saturated hydrocarbons all carbon atoms are
  connected together with single bonds
• Unsaturated hydrocarbons involve one or more
  multiple (double, triple) C-C bonds
• Hydrocarbon derivatives contain carbon and
  hydrogen, and one or more other elements (P, N,
  O, Cl, etc.)

5
Hydrocarbons and hydrocarbon derivatives

• Saturated hydrocarbons may be found in two
  possible formats

6
Alkanes acyclic saturated hydrocarbons

• An alkane is a saturated hydrocarbon that is
  acyclic (does not possess ring-structure).
• Because all C-C bonds are single bonds (and
  because the other bonds that carbon needs to get
  an octet are to H-toms), alkanes have the general
  formula CnH2n2 (n of C-atoms)

Examples of alkanes CH4 C2H6 C3H8
7
Alkanes acyclic saturated hydrocarbons

• In an alkane, each carbon is tetrahedral (it has
  four bonds to other atoms. Rem VSEPR)

CH4
C2H6
C3H8
8
Alkanes acyclic saturated hydrocarbons

• Chemical formulas for alkanes are written as
  CnH2n2 however, structural formulas give more
  information.
• Chemical formula reveals the type and number of
  each element in the compound
• Structural formulas show how each atom in the
  molecule is connected

expanded structural formula
condensed structural formula
name
9
Alkanes acyclic saturated hydrocarbons

• For longer carbon chains, an abbreviated,
  condensed structural formula is advantageous, as
  it shows most of the information of the expanded
  formula without taking up as much space

6 CH2- units between two CH3-groups
An 8-carbon chain (two CH3-groups linked by a
6-carbon chain (6-CH2- units)
10
Alkanes acyclic saturated hydrocarbons

• Sometimes, a simple skeletal structural formula
  can be used to convey hydrocarbon structure

mean the same thing
11
Alkane isomerism

• The types of alkanes weve considered so far
  involve straight chain types, where the carbon
  atoms form a continuous series (i.e. no
  branches).
• When alkanes having four or more carbons are
  considered, there is more than one structural
  formula that can be used to describe a given
  molecular formula.

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Alkane isomerism

• The formula C4H10 can be represented by the
  following condensed structural formulas

These compounds possess the same chemical
formula, but differ in the way the atoms are
arranged (isomers)
13
Alkane isomerism

• The C4H10 shown on the left is called a
  continuous chain alkane (or an unbranched/straigh
  t-chain alkane).
• The one on the right is called a branched-chain
  alkane

These are called constitutional isomers which
differ in their atom-to-atom connectivity
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Alkane isomerism

• As the number of carbon atoms in the alkane
  grows, so do the number of possible isomers.

15
Conformations of alkanes

• The carbon-carbon bonds in alkanes permit
  rotation of each carbon-group with respect to the
  others that are chemically bound to it.

16
Conformations of alkanes

• Conformations are specific, 3-dimensional
  arrangements of atoms in organic molecules (at
  some instant) that result from rotation about C-C
  single bonds.
• Several conformations of a six-carbon chain are
  shown using the skeletal structures below

All the same molecule C6H14
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Conformations of alkanes

• Note that the following two skeletal structures
  describe two different alkanes

Alkane on the left is a 6-carbon, continuous
chain structure. Alkane on the right is a
branched structure (a 5-carbon, continuous chain
that has a 1-carbon branch)
18
Conformations of alkanes

• Do the following pairs of condensed structural
  formulas describe the same alkane?

19
IUPAC nomenclature for alkanes

• The names that have been shown for the branched
  alkanes so far are common names (made as these
  compounds were identified).
• As the number of organic compounds catalogued
  grew, a system for naming was developed by the
  International Union of Pure and Applied Chemistry
  (IUPAC).
• The basic system used is one that employs a
  prefix-type name.

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IUPAC nomenclature for alkanes

• Names for continuous chain alkanes (first ten)
  are shown below. The names use a prefix (e.g.
  meth-) to designate the number of carbon atoms in
  the chain.

Alkanes have ane at the end of their name
Prefix
Meth- Eth- Prop- But- Pent- Hex- Hept- Oct- Non- D
ec-
21
IUPAC nomenclature for alkanes

• Branched-chain alkanes can be described as
  continuous-chain alkanes with branches
  (substituents).
• The IUPAC system of naming branched-chain alkanes
  describes the type and location of substituents
  before the name of the longest, continuous chain
  of carbon atoms in the alkane.

Substituents are the branches in branched-chain
alkanes. They are atoms (or groups of atoms for
the kind well look at first) that hang off the
main carbon chain.
22
IUPAC nomenclature for alkanes

• Substituents in branched-chain alkanes are called
  alkyl groups. An alkyl group is the group of
  atoms that would be created by removing a
  hydrogen atom from an alkane. They are named
  according to the alkane from which they are
  derived.

To get the substituent name take the alkane name
and replace the ane part with yl
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IUPAC nomenclature for alkanes

• To name a branched alkane, follow these steps
• 1) Identify the longest, continuous carbon chain
  in the structure. This will be the base of the
  branched alkanes name.

So far, we know this compound is going to be
called some kind of butane)
24
IUPAC nomenclature for alkanes

• 2) Number this chain in a way that gives the
  carbon(s) with the substituent the lowest
  possible, overall numbering.
• The methyl substituent is thus located on C-2
  (carbon-2)

(something something butane)
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IUPAC nomenclature for alkanes

• After locating the alkyl substituent by number,
  prefix the parent-chain alkane name (the longest,
  continuous carbon chain) with the number and the
  name of the substituent

2-Methylbutane
Separate the number from the substituent name
with a hyphen, and the last substituent name
reads directly into the parent chain alkane name
26
IUPAC nomenclature for alkanes

• Another example

1. Find the longest, continuous chain of C-atoms
2. Number them in a way that gives all substituents
   the lowest total numbering
3. Prefix the name of the parent alkane with the
   number and name of the substituent

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IUPAC nomenclature for alkanes

• One with multiple substituents

2,3,4-Trimethylhexane
In cases where multiple substituents of the same
type are present, prefix the substituent name
with di-, tri-, tetra-, etc. to indicate how many
of them are present
28
IUPAC nomenclature for alkanes

• If more than one kind of substituent is present,
  the alphabetic order of the substituents take
  priority over the number of the substituent when
  numbering the parent chain

Separate different substituents with hyphens
3-Ethyl-4,5-dipropyloctane
3-Ethyl-2-methylhexane (not 2-Methyl-3-ethylhexane
)
The prefix part of the propyl substituents are
not counted for alphabetical ordering
(come back to this structure later)
29
IUPAC nomenclature for alkanes

• IUPAC punctuation rules
• Separate numbers from letters with hyphens
• Separate numbers from other numbers with commas
• Dont separate the last substituent name from the
  parent alkane chain

2
4-Ethyl-2,3-dimethyl-5-propylnonane
3
1
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Line-angle structural formulas for alkanes

• Line-angle structural formulas describe
  carbon-carbon bonds with straight lines (each
  point in the diagram represents a carbon atom
  with four bonds to carbon(s) and hydrogen(s)
  around it)

It is understood that each C-atom has four bonds
C-H bonds are there, but not shown
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Classification of carbon atoms

• The carbon atoms in organic structures are
  classified as primary, secondary, tertiary, or
  quaternary, depending on the number of other
  carbon atoms bound to them.
• Primary (1o) C bounds to one other C-atom
• Secondary (2o) C bound to two other C-atoms
• Tertiary (3o) C bound to three other C-atoms
• Quaternary (4o) C bound to four other C-atoms

1o
3o
4o
2o
32
Branched-chain alkyl groups

• Sometimes, branched-chain substituents are
  encountered. These are named according to the
  parent alkane from which they are derived.

Substituent derives from a 4-C alkane (butane)
and point of attachment is a tertiary C of the
alkane
Substituent derives from a 3-C alkane (propane)
and point of attachment is a secondary C of the
alkane
4-Isopropyloctane
4-tert-Butyloctane
could also call this 4-sec-Propyloctane
33
Branched-chain alkyl groups
another point for the purposes of
capitalization (at the beginning of the name),
tert- and sec-are not capitalized, but iso is
4-Isopropyloctane
4-tert-Butyloctane
34
Branched-chain alkyl groups

• Given a choice between unbranched substituents
  and branched substituents, use unbranched ones
  for naming

3-Ethyl-2-methylhexane (not 2-Methyl-3-ethylhexane
)
also, not 3-Isopropylhexane
35
Cycloalkanes

• Cyclic alkanes (cycloalkanes) are alkane chains
  where the end carbons are linked together (need
  to kick off 2 H atoms from the formula of the
  corresponding straight-chain alkane to get the
  cycloalkane formula).
• The general formula for a cycloalkane is CnH2n

36
Cycloalkanes
37
IUPAC nomenclature for substituted cycloalkanes

• If one substituent exists on a cycloalkane, no
  numbering is needed to denote its location

Ethylcyclohexane
38
IUPAC nomenclature for substituted cycloalkanes

• If two substituents are present, the ring is
  numbered follows alphabetic priority.
• If more than two substituents are present, the
  ring numbering is assigned in a way that gives
  the lowest overall substituent numbers (order
  they are reported in is still alphabetic)

1-Ethyl-2-methylcyclohexane
2-Ethyl-1-methyl-4-propylcyclohexane (not
1-Ethyl-2-methyl-5-propylcyclohexane or
1-Methyl-2-ethyl-4-propylcyclohexane)
39
Isomerism in cycloalkanes

• Constitutional isomers are possible for
  cycloalkanes having four or more carbons

These isomers differ in the way the carbon
atoms are connected together (constitutional
isomers)
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Isomerism in cycloalkanes

• As before, as the number of carbons in the
  (cyclo)alkane grows, so do the number of
  constitutional isomers.

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Isomerism in cycloalkanes

• Another kind of isomerism we havent yet
  encountered, called stereoisomerism, involves
  molecules that have the same molecular formula,
  same atom-to-atom connectivity, but differ in the
  3-dimensional arrangement of the atoms in space.
• In cycoalkanes, there may exist the possibility
  of cis-, trans- isomers

42
Isomerism in cycloalkanes

• There are two distinct molecules. One cant be
  converted into the other without breaking bonds
  first.
• Can have this form of isomerism for any
  cycloalkane that has more than one substituent.

43
Isomerism in cycloalkanes

• Substituents also dont need to be on adjacent
  carbon atoms of the ring (but cant be on the
  same carbon atom of the ring)

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Sources of alkanes and cycloalkanes

• The crude petroleum that is obtained at drilling
  sites is a mixture of hydrocarbons (cyclic and
  acyclic) that is purified (refined) by taking
  advantage of the boiling point differences of the
  various components

45
Sources of alkanes and cycloalkanes

• Boiling point is observed to increase with
  increasing chain C-chain length (and ring size
  for cycloalkanes).
• About a 30o increase per additional C (CH2-
  unit) in the chain.

46
Physical properties of alkanes and cycloalkanes

1. Alkanes and cycloalkanes are water-insoluble
2. Alkanes and cycloalkanes have densities that are
   less than that of water (0.6 0.8 g/mL, as
   compared to 1 g/mL for H2O)
3. Boiling points of continuous chain alkanes and
   cycloalkanes increase with an increase in
   carbon-chain length or ring size

• Cycloalkanes have higher boiling points than
  corresponding alkanes because they are more
• rigid
• Branched chain alkanes have lower boiling points
  because they are more compact and have
• less surface areas that straight-chain forms

47
Chemical properties of alkanes and cycloalkanes

• Alkanes and cycloalkanes have low chemical
  reactivities. The C-C bonds and C-H bonds are
  non-polar, which do not encourage reactions with
  other species, and the bond strengths are fairly
  high (strong bonds)
• Two reactions that they are susceptible to are
  combustion and halogenation

48
Chemical properties of alkanes and cycloalkanes

• In a combustion reaction, alkanes and
  cycloalkanes are reacted with O2 to form CO2 in
  an oxygen-rich environments (or CO or other
  C-products in less O2-rich environments).
• Some examples of alkane combustion reactions
• CH4 2O2 ? CO2 2H2O heat
• 2C6H14 19O2 ? 12CO2 14H2O heat

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Chemical properties of alkanes and cycloalkanes

• Halogenation reactions involve halogen atoms
  (group 7 F, Cl, Br, I). These reactions involve
  the substitution of a hydrogen atom of an alkane
  with a halogen

In general, the reaction goes like this
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Chemical properties of alkanes and cycloalkanes

• In a substitution reaction, an atom or group of
  atoms is replaced by another atom (or group of
  atoms) in a hydrocarbon or hydrocarbon
  derivative.
• On the surface, they are like the replacement
  reactions weve already studied

51
Chemical properties of alkanes and cycloalkanes

• In situations where more than one type of product
  can result, a mixture of products is usually
  obtained.
• In many cases, more than one H-atom can be
  substituted

52
Nomenclature and properties of halogenated alkanes

• Halogenated alkanes (or haloalkanes) are
  hydrocarbons (or their derivates) that possess at
  least one halogen atoms
• Naming rules
• Halogens are treated just like other (alkyl)
  substituents when numbering and alphabetic naming
  are considered
• Substituents are called fluoro-, chloro, bromo-,
  and iodo- for the purposes of assigning names

53
Nomenclature and properties of halogenated alkanes

• In terms of chemical reactivity, halogenated
  alkanes are more reactive than alkanes and
  cycloalkane analogues, because the C-X bond (X
  halogen) makes the bond polar and thus
  susceptible to reactions that require initial
  dipole-dipole interactions.
• Some halogenated alkanes have densities greater
  than that of water.
• Chloroalkanes with 2 or more Cl-atoms
• Bromoalkanes
• Iodoalkanes