Saturated hydrocarbons
Chapter 12
Coming up…
• Ch-12: saturated hydrocarbons (alkanes, cycloalkanes)
• Ch-13: Unsaturated hydrocarbons (alkenes, alkynes,
aromatics)
• Ch-14: Alcohols, phenols, ethers
• Ch-15: Aldehydes and ketones
• Ch-16: Carboxylic acids, esters, acid derivatives
• Ch-17: Amines and amides
• Ch-18: Carbohydrates
• Ch-19: Lipids
• Ch-20: Proteins
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 10 million organic
compounds catalogued
• Inorganic chemistry concerns the chemistry of
the other 117 elements. Around 1.7 million
of these are known
Non-polar
hydrocarbon tail
~1.7 million
A polar,
charged group
~10 million
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.
C
four
single
bonds
C
double bond,
2 single
bonds
C
triple bond,
single bond
two double bonds
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.)
Hydrocarbons and hydrocarbon
derivatives
• Saturated hydrocarbons may be found in two
possible formats:
an acyclic, 6-C chain
a cyclic 6-C structure
carbon skeletal structures
(these are not complete Lewis structures)
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 CnH2n+2 (n = # of C-atoms)
Examples of alkanes:
CH4
C2H6
C3H8
Alkanes: acyclic saturated
hydrocarbons
• In an alkane, each carbon is tetrahedral (it has
four bonds to other atoms. Rem: VSEPR)
CH4
C2H6
C3H8
Alkanes: acyclic saturated
hydrocarbons
• Chemical formulas for alkanes are written as CnH2n+2; 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
CH4
CH3-CH3
CH3-CH2-CH3
CH4
C2H6
C3H8
name
methane
ethane
propane
molecular
formula
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
CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3
CH3-(CH2)6-CH3
An 8-carbon chain (two CH3-groups
linked by a 6-carbon chain (6-CH2- units)
6 –CH2- units between
two CH3-groups
Alkanes: acyclic saturated
hydrocarbons
• Sometimes, a simple skeletal structural
formula can be used to convey hydrocarbon
structure (these are usually not used)
mean
the same
thing
Alkane isomerism
• The types of alkanes we’ve 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.
Alkane isomerism
• The formula C4H10 can be represented by the
following condensed structural formulas:
butane
(or n-butane)
isobutane
C4H10
These compounds possess the same chemical formula,
but differ in the way the atoms are arranged (isomers)
“isotopes” sounds like “isomers” – they don’t mean the same thing.
Isotopes are nuclides of the same element that possess different numbers of neutrons
(e.g. 12C and 13C).
Alkane isomerism
• The C4H10 shown on the left is called a continuous
chain alkane (or an unbranched/”straight-chain”
alkane).
• The one on the right is called a “branched-chain”
alkane
butane
(or n-butane)
isobutane
These are called “constitutional isomers” which differ in their atom-to-atom connectivity
In every case, the two
(or more) molecules that
are being compared have
the same molecular
formula, but differ in their
structures, somehow.
Alkane isomerism
• As the number of carbon atoms in the alkane
grows, so do the number of possible isomers.
C5H12
pentane
isopentane
neopentane
Conformations of alkanes
all single bonds
• The carbon-carbon bonds in alkanes permit
rotation of each carbon-group with respect to
the others that are chemically bound to it.
C4H10
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
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)
Conformations of alkanes
• Do the following pairs of condensed structural
formulas describe the same alkane?
a)
b)
c)
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.
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
Memorize
these
prefixes
the end of their name
Prefix
MethEthPropButPentHexHeptOctNonDec-
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
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 we’ll look at first) that hang off the main carbon chain.
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.
methane
a "methyl" substituent
To get the substituent name:
take the alkane name and
replace the “ane” part with “yl”
propane
a "propyl" substituent
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
alkane’s name.
a 4-carbon, continuous chain
So far, we know this compound is going to be called some kind of butane
IUPAC nomenclature for alkanes
2) Number this chain in a way that gives the
carbon(s) with the substituent the lowest possible,
overall numbering.
1
2
3
4
The methyl substituent is thus located on C-2
(carbon-2)
(something something butane)
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:
1
2
3
4
The “2” in this case is actually
redundant, because if a
methyl substituent were on
any other carbon except
carbon #2, the longest chain of
carbon atoms would be 5,
making the molecule a pentane.
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
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
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
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-2-methylhexane
(not 2-Methyl-3-ethylhexane)
(come back to this structure later)
3-Ethyl-4,5-dipropyloctane
The prefix part of the propyl substituents
are not counted for alphabetical ordering
IUPAC nomenclature for alkanes
• IUPAC punctuation rules:
1. Separate numbers from letters with hyphens
2. Separate numbers from other numbers with
commas
3. Don’t separate the last substituent name from
the parent alkane chain
2
4-Ethyl-2,3-dimethyl-5-propylnonane
1
3
Line-angle structural formulas for
alkanes
• Line-angle structural formulas describe carboncarbon 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
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
4o
2o
3o
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
Substituent derives from a 3-C alkane
(propane) and point of attachment
is a secondary C of the substituent
4-Isopropyloctane
could also call this 4-sec-Propyloctane
(butane) and point of attachment
is a tertiary C of the substituent
4-tert-Butyloctane
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
Branched-chain alkyl groups
• Given a choice between unbranched
substituents and branched substituents,
should use unbranched ones for naming
Preferable to call this 3-Ethyl-2-methylhexane
also, not 3-Isopropylhexane
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
Cyclopropane
Cyclooctane
Cyclohexane
Cyclobutane
Cyclopentane
Cycloheptane
=
C6H12
C6H12
Cyclononane
Cycloalkanes
IUPAC nomenclature for substituted
cycloalkanes
• Naming: If one substituent exists on a
cycloalkane, no numbering is needed to
denote its location
Ethylcyclohexane
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)
always ensure the
lowest overall
numbering is followed
1-Ethyl-2-methylcyclohexane
2-Ethyl-1-methyl-4-propylcyclohexane
(not 1-Ethyl-2-methyl-5-propylcyclohexane
or 1-Methyl-2-ethyl-4-propylcyclohexane)
Isomerism in cycloalkanes
• Constitutional isomers are possible for
cycloalkanes having four or more carbons:
C4H8
Cyclobutane
C4H8
Methylcyclopropane
These isomers differ in the way the carbon atoms
are connected together (constitutional isomers)
Isomerism in cycloalkanes
• As before, as the number of carbons in the
(cyclo)alkane grows, so do the number of
constitutional isomers.
C5H10
Cyclopentane
C5H10
Methylcyclobutane
C5H10
1,2-Dimethylcyclopropane
C5H10
Ethylcyclopropane
Isomerism in cycloalkanes
• Another kind of isomerism we haven’t 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
cis-1,2-Dimethylcyclopentane
CH3-substituents are
both above the C-C
bond of cyclopentane
trans-1,2-Dimethylcyclopentane
One CH3-substituent is
above the C-C bond of
cyclopentane and the
other one is below it
Isomerism in cycloalkanes
• There are two distinct molecules. One can’t be converted into
the other without breaking bonds first.
• Can have this form of isomerism for any cycloalkane that has
more than one substituent.
trans-1-Ethyl-2-methylcyclohexane
cis-1-Ethyl-2-methylcyclohexane
Isomerism in cycloalkanes
• Substituents also don’t need to be on adjacent
carbon atoms of the ring (but can’t be on the same
carbon atom of the ring)
trans-1-Ethyl-3-methylcyclohexane
cis-1-Ethyl-3-methylcyclohexane
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
Sources of alkanes and
cycloalkanes
• Boiling point is observed to increase with increasing chain Cchain length (and ring size for cycloalkanes).
• About a 30o increase per additional C (–CH2- unit) in the chain.
Increasing
London forces
of attraction
Physical properties of alkanes and
cycloalkanes
1.
2.
3.
Alkanes and cycloalkanes are waterinsoluble
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)
Boiling points of continuous chain alkanes
and cycloalkanes increase with an
increase in carbon-chain length or ring
size
Boiling point for alkanes having a given number of C-atoms: cycloalkane > “straight” chain > branched chain
•
•
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 (less interaction with other, similar molecules) than straight-chain forms
Why is something like this important in life science?
• Cholesterol is used by the body for
–
–
–
–
•
building cell membranes
making bile
vitamin D
steroid horomones
cholesterol,
(mainly a hydrocarbon)
Cholesterol is transported through the bloodstream by
lipoproteins. These shell structures have a water-unfriendly
core and a water-friendly exterior
Low-Density Lipoprotein
•
•
LDL is a lipoprotein that has mostly cholesterol content.
HDL has mostly protein content
High-Density Lipoprotein
LDL is used to transport cholesterol to sites in the body,
through the bloodstream.
http://www.answers.com/topic/lipoprotein
Why is something like this important in life science?
• If LDL transports cholesterol to a site
(e.g. tissue) that doesn’t require
cholesterol, the LDL remains in the
bloodstream, eventually releasing its
contents.
• Cholesterol molecules aren’t watersoluble, so they aggregate, forming
insoluble clumps that collect along the
arterial walls (atherosclerosis).
Main forces of attraction between
cholesterol molecules are London forces,
which increase with molecular weight.
• This restricts blood flow and can
eventually create a blockage
(stroke/heart attack).
Essentials of general, organic, and biochemistry.
D. Guinn, R. Brewer, W.H. Freeman, NY, 2010.
Why is something like this important in life science?
• The condition (depending on the stage) can be
corrected by:
• diet/exercise
• medication
• angioplasty
Essentials of general, organic, and biochemistry.
D. Guinn, R. Brewer, W.H. Freeman, NY, 2010.
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
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 O2rich environments).
• Some examples of alkane combustion reactions:
CH4 + 2O2  CO2 + 2H2O + heat
2C6H14 + 19O2  12CO2 + 14H2O + heat
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 iodofor the purposes of assigning names
2-Chloro-3-methylbutane
3-Bromo-1-chlorobutane
1-Ethyl-2-fluorocyclohexane
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.