3. Organic Compounds: Alkanes and Cycloalkanes
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Transcript 3. Organic Compounds: Alkanes and Cycloalkanes
Organic Chemistry
M. R. Naimi-Jamal
Faculty of Chemistry
Iran University of Science & Technology
Chapter 2.
Alkanes and Cycloalkanes
Based on: McMurry’s Fundamental of Organic Chemistry, 4th
edition, Chapter 2
Families of Organic Compounds
Organic compounds can be grouped into families by
their common structural features
We shall survey the nature of the compounds in a tour
of the families in this course
This chapter deals with alkanes, compounds that
contain only carbons and hydrogens, all connected
exclusively by single bonds
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2.1 Functional Groups
Functional group - collection of atoms at a site
within a molecule with a common bonding pattern
The group reacts in a typical way, generally
independent of the rest of the molecule
For example, the double bonds in simple and
complex alkenes react with bromine in the same
way
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Double Bond as Functional Group
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Survey of Functional Groups
Table 3.1 lists a wide variety of functional
groups that you should recognize
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
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Types of Functional Groups: Multiple
Carbon–Carbon Bonds
Alkenes have a C-C double bond
Alkynes have a C-C triple bond
Arenes (or aromatic hydrocarbons) have special
bonds that are represented as alternating single and
double C-C bonds in a six-membered ring
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Multiple Carbon–Carbon Bonds
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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)
Bonds are polar, with partial positive charge on C (+) and
partial negative charge () on electronegative atom
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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: C-O bonded to the C=O
Amide: C-N bonded to the C=O
Acid chloride: Cl bonded to the C=O
Carbonyl C has partial positive charge (+)
Carbonyl O has partial negative charge (-).
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2.2 Alkanes and Alkane Isomers
Alkanes: Compounds with C-C single bonds and C-H
bonds only (no functional groups)
The formula for an alkane with no rings (acyclic) must
be CnH2n+2 where the number of C’s is n
Alkanes are saturated with hydrogen (no more can be
added)
They are also called aliphatic compounds
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A “saturated” fat (glyceryl stearate):
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Alkane Isomers
CH4 = methane, C2H6 = ethane, C3H8= propane
The molecular formula of an alkane with more
than three carbons can give more than one
structure
C4 (butane) = butane and isobutane
C5 (pentane) = pentane, 2-methylbutane
(isopentane), and 2,2-dimethylpropane
(neopentane)
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Methane, ethane, & propane
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Butanes (C4H10):
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Pentanes (C5H12):
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Alkane
Isomers
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Alkane Isomers
Alkanes with C’s connected to no more
than 2 other C’s are straight-chain or
normal alkanes
Alkanes with one or more C’s connected to
3 or 4 C’s are branched-chain alkanes
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Constitutional (Structural) Isomers
Isomers that differ in how their atoms are
arranged in chains are called constitutional
(or structural) isomers
Compounds other than alkanes can also be
constitutional isomers of one another
They must have the same molecular formula
to be isomers
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Constitutional Isomers
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Condensed Structures of Alkanes
We can represent an alkane in a brief form or in many types of
extended form
A condensed structure does not show bonds but lists atoms,
such as
CH3CH2CH3 (propane)
CH3(CH2)2CH3 (2,2-dimethylpropane)
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Names of Small Hydrocarbons
No. of Carbons
Formula Name
(CnH2n+2)
1
Methane
CH4
2
Ethane
C2H6
3
Propane
C3H8
4
Butane
C4H10
5
Pentane
C5H12
6
Hexane
C6H14
7
Heptane
C7H16
8
Octane
C8H18
9
Nonane
C9H20
10
Decane
C10H22
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2.3 Alkyl Groups
Alkyl group – remove one H from an alkane (a part of
a structure)
General abbreviation “R” (for Radical, an incomplete
species or the “rest” of the molecule)
Name: replace -ane ending of alkane with -yl ending
-CH3 is “methyl” (from methane)
-CH2CH3 is “ethyl” from ethane
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2.3 Alkyl Groups
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Alkyl Groups
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Types of Alkyl groups
a carbon at the end of a chain (primary alkyl
group)
a carbon in the middle of a chain (secondary
alkyl group)
a carbon with three carbons attached to it
(tertiary alkyl group)
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Types of Carbon Atoms
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Types of Functional groups
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Types of Hydrogens:
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2.4 Naming Alkanes: IUPAC
Compounds are given systematic names by a
process that uses:
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2.4 Naming Alkanes: IUPAC
Follows specific rules
Named as longest contiunous chain of C’s
Carbons in that chain are numbered in sequence
Substituents are numbered at their point of
attachment
Compound name is one word (German style)
Complex substituents are named similarly
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1. Find the Parent: longest continuous carbon
chain
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2. Number the atoms in the chain with the end of
the chain nearer the first substituent
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3. Identify & number the substituents. When two
substituents are present at an equal distance from either end of
the longest chain, we number the ring beginning with the
substituent first in the alphabet. In deciding on alphabetical
oreder disregard multiplying prefixes such as “di” and “tri”
etc. and the prefixes like sec- or tert.
H3C CH2 CH CH2 HC
H3C
CH2 CH3
CH2
H3C
CH3
3-Ethyl-5-methylheptane
H3C CH2 CH CH2 HC
CH2 CH3
CH2
CH3
NOT
3-Methyl-5-ethylheptane
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4. Wenn three or more substituents are present, we begin at
the substituent that leads to the lowest set of locants.
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4. Write the name
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Practice problem 2.2: IUPAC name?
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Solution:
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Practice prob. 2.3: structure?
3-isopropyl-2-methylhexane
C-C-C-C-C-C
Two substituents: isopropyl & methyl
Add hydrogens to complete the structure
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Solution:
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Problem 2.11: IUPAC names?
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10.1 Naming Alkyl Halides
Name is based on longest carbon chain
(Contains double or triple bond if present)
Number from end nearest any substituent
(alkyl or halogen)
Halogens have same priority as alkyl groups
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Naming with Multiple Halides
If more than one of the
same kind of halogen is
present, use prefix di,
tri, tetra
If there are several
different substituents
(halogens or alkyls),
number them and list
them in alphabetical
order
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Naming if Two Halides or Alkyl Are Equally
Distant from Ends of Chain
Begin at the end nearer the substituent whose
name comes first in the alphabet
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2.5 Properties of Alkanes
Called paraffins (low affinity compounds)
because they are relatively unreactive
They will burn in a flame, producing carbon
dioxide, water, and heat
CH4 + 2 O2 → CO2 + 2 H2O + heat
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2.5 Properties of Alkanes
They react with Cl2 in the presence of light to
replace H’s with Cl’s (not easily controlled)
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Physical Properties
Boiling points and melting points increase as
size of alkane increases
Forces between molecules (temporary dipoles,
dispersion) are weak
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Physical Properties
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2.6 Cycloalkanes
Cycloalkanes are alkanes that have carbon
atoms that form a ring (called alicyclic
compounds)
Simple cycloalkanes are rings of CH2
units, (CH2)n, or CnH2n
Structure is shown as a regular polygon with
the number of vertices equal to the number of
C’s (a projection of the actual structure)
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2.6 Cycloalkanes
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Complex Cycloalkanes
Naturally occurring materials contain
cycloalkane structures
Examples:
chrysanthemic acid (cyclopropane),
prostaglandins (cyclopentane),
steroids (cyclohexanes and cyclopentane)
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Complex Cycloalkanes
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Properties of Cycloalkanes
Melting points are
affected by the
shapes and the way
that crystals pack so
they do not change
uniformly
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2.7 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 point of
attachment as C1 and number the substituents on the ring so
that the second substituent has as low a number as possible.
When two substituents are present, we number the ring
beginning with the substituent first in the alphabet.
Wenn three or more substituents are present, we begin at the
substituent that leads to the lowest set of locants
Number the substituents and write the name
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1. Find the parent:
or butylcyclopropane
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Number the
substituents &
write the name:
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Examples:
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Problem 2.15: IUPAC names?
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2.8 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)
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2.8 Cis-Trans Isomerism in Cycloalkanes
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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
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Stereoisomers
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Practice Prob. 2.4: Name?
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Problem 2.18: IUPAC Name?
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Gasoline: Octane Ratings
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Chapter 2, Questions
30, 34, 35, 36, 37
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