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
3
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
4
5
Double Bond as Functional Group
6
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
7
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
8
Multiple Carbon–Carbon Bonds
9
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
10
11
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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13
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
14
A “saturated” fat (glyceryl stearate):
15
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)
16
Methane, ethane, & propane
17
Butanes (C4H10):
18
Pentanes (C5H12):
19
Alkane
Isomers
20
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
21
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
23
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
25
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
27
Alkyl Groups
28
29
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)
30
Types of Carbon Atoms
31
Types of Functional groups
32
Types of Hydrogens:
33
2.4 Naming Alkanes: IUPAC

Compounds are given systematic names by a
process that uses:
34
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
35
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
37
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
38
4. Wenn three or more substituents are present, we begin at
the substituent that leads to the lowest set of locants.
39
4. Write the name
40
Practice problem 2.2: IUPAC name?
41
Solution:
42
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
43
Solution:
44
Problem 2.11: IUPAC names?
45
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
46
47
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
48
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
50
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
52
Physical Properties
53
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)
56
Complex Cycloalkanes
57
Properties of Cycloalkanes
Melting points are
affected by the
shapes and the way
that crystals pack so
they do not change
uniformly
58
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
60
Number the
substituents &
write the name:
61
Examples:
62
Problem 2.15: IUPAC names?
63
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)
64
2.8 Cis-Trans Isomerism in Cycloalkanes
65
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
66
Stereoisomers
67
Practice Prob. 2.4: Name?
68
Problem 2.18: IUPAC Name?
69
Gasoline: Octane Ratings
70
Chapter 2, Questions
30, 34, 35, 36, 37
71