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CHEM 1AA3 Lecture Notes - Eclipsed Conformation, Staggered Conformation, Alkane


Department
Chemistry
Course Code
CHEM 1AA3
Professor
Pippa Lock

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Chapter 26: Structure of Organic Compounds
Organic Compounds and Structures: An Overview
The simplest organic compounds are those of carbon and hydrogen hydrocarbons
and the simplest hydrocarbon is methane
The three-dimensional structure of hydrocarbons can be shown by the structural
formula, the condensed structural formula, the ball-and-stick model, the space filling
model and the dashed-wedged line notation
Constitutional Isomerism in Organic Compounds
o Compounds that have the same molecular formula, but different structural
formulas are called isomers
o Constitutional isomers have different bond connectivities and thus different
skeletal structures
o Ex. Butane vs. methylpropane
Nomenclature
o Saturated hydrocarbons are those with all carbon-to-carbon bonds as single
bonds (alkanes)
o A side chain is an alkane with one hydrogen atom removed
The resulting group of atoms is called an alkyl group (methyl, propyl)
o A primary carbon is attached to one other carbon atom
Carbon atoms at the end of an alkane chain are always primary carbons
The hydrogen atoms attached to a primary carbon atom are called
primary hydrogen atoms
An alkyl group formed by removing a primary hydrogen is a primary
group
o A secondary carbon is attached to two other carbon atoms
o A tertiary carbon is attached to three other carbon atoms
o A quaternary is a carbon attached to four carbon atoms
o The following rules enable us to name branched-chain hydrocarbons as long as
we apply the rules in sequence
Select the longest continuous carbon chain in the molecule and use the
hydrocarbon name of this chain as the base name
Consider every branch of the main branch to be a substituent alkyl group
Number the C atoms of the continuous base chain so that the
substituents appear at the lowest numbers possible
Separate numbers from one another with commas, but no spaces and
separate numbers from letters with hyphens
List the substituents alphabetically by name
When determining alphabetical order, the prefixes di-, tri-, sec-
and tert- are ignored
Therefore tert-butyl precedes methyl in 4-tert-butyl-2-
methylheptane
The prefix iso- is not ignored
Functional Groups

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o Organic compounds typically contain elements in addition to carbon and
hydrogen
o These elements occur as distinctive groupings of one or several atoms called
functional groups
o Compounds with the same functional group generally have similar chemical
properties
Alkanes
The essential characteristic of alkane hydrocarbon molecules is that they have only
single covalent bonds
The bonds in these compounds are said to be saturated
Substances whose molecules differ only by a constant unit (-CH2) are said to form a
homologous series
Members of such series usually have closely related chemical and physical properties
Intermolecular attractions between the straight-chain molecules are strongest and
these molecules have the highest BP
Isomers with more compact structures have lower BP
Conformations
o Conformations are different spatial arrangements that are possible in a molecule
o One conformation can be converted into another by rotations about bonds
o Newman projections are used to represent the many different spatial
arrangements of atoms that result from rotations around bonds
o A staggered conformation is when the carbon-hydrogen bonds in one CH3
group are positioned exactly halfway between those of the other CH3 group and
the H atoms are located a maximum distance apart
o The eclipsed conformation is when all the hydrogen atoms on the first carbon
atom are directly in front of those on the second carbon atom
o The eclipsed and staggered conformations represent two extremes and all the
possible conformations in between are collectively called skew conformations
o The difference in energy between the eclipsed and staggered conformations is
called the rotational or torsional energy
o The totally eclipsed conformation is highest in energy because in this
conformation, the two largest constituents are eclipsed and interfere with each
other
The interference or crowding of large substituents is called steric
hindrance
o The anti conformation is a staggered conformation when the substituents are
farthest apart and it has the lowest energy of all the conformations
o The other staggered conformations has the substituents to the left or and right
of each other and is called the gauche conformation
o All eclipsed conformations have higher energy than all staggered conformations
All the close atoms contribute to high energy
o Conformation vs. Configuration

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Conformation refers to the arrangement of atoms in a molecule that can
be changed by simple rotation of single bonds, without breaking any
bonds
Configuration is the permanent geometry of a molecule resulting from
the spatial arrangement of its bonds
Must break bonds to change configuration
Preparation of Alkanes
o The main source of alkanes is petroleum
o In the presence of a metal catalyst such as Pt, Pb or Ni, unsaturated
hydrocarbons, whether containing double or triple bonds, may be converted to
alkanes by the addition of H atoms to the multiple bond systems
o In another type of reaction, halogenated hydrocarbons react with alkali metals
to produce alkanes of double the carbon content
o Another method is to have alkali metal salts of carboxylic acids fuse with alkali
metal hydroxides
Isomers and Cycloalkanes
Skeletal or structural isomers have
o The same molecular formula, but different connectivity
o Different physical properties
Cycloalkanes
o CnH2n
o Cyclo-propane/butane/pentane/hexane..
o Molecules are not always flat
Ex. Cyclohexane can form the boat or chair conformation
The chair conformation is more favourable because the electron cloud is
more spaced out
o In cycloalkanes, there are equatorial substituents and axial substituents
Equatorial substituents are more favourable than axial substituents
o Ring Strain
In propane, the bond angles of the carbon atoms are 109.50C
In cyclopropane, the bond angles of the carbon atoms are 600C
Also, the H atoms are eclipsed
Not very favourable, has high energy
Alkenes and Alkynes
Hydrocarbons whose molecules contain some double or triple bonds between C atoms
are said to be unsaturated
If the molecule has one double bond, they hydrocarbons are simple alkenes and have
the general formula CnH2n
Simple alkynes have one triple bond in their molecules and have the general formula
CnH2n-2
There are some modifications of rules required to name alkenes and alkynes
o Select as the base chain the longest chain containing the multiple bond
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