Download 2.12 Noncovalent Interactions between Molecules Noncovalent

1/31/2011
2.12 Noncovalent Interactions between
Molecules
Noncovalent Interactions
• Also called intermolecular forces or van der Waals forces
• Types of noncovalent interactions include:
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Dipole-Dipole forces
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Occur between polar molecules as a result of electrostatic
interactions among dipoles
Forces are either attractive or repulsive
•
Attractive geometry is lower in energy and therefore predominates
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Attractive
Repulsive
Noncovalent Interactions between Molecules
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Dispersion forces
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Attractive dispersion forces in nonpolar molecules are caused by
temporary dipoles
One side of the molecule may have a slight excess of electrons
relative to the opposite side, giving the molecule a temporary dipole
Temporary dipole in one molecule causes a nearby molecule to
adopt a temporarily opposite dipole resulting in a small attraction
between the two molecules
Arise because the electron distribution within molecules is constantly
changing
Noncovalent Interactions between Molecules
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Hydrogen Bonds
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A weak attraction between a hydrogen atom bonded to an
electronegative O or N and an electron lone pair on another O
or N atom
Strong dipole-dipole interaction involving polarized O-H and
N-H bonds
Important noncovalent interaction in biological molecules
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Noncovalent Interactions between Molecules
•
Effects of Hydrogen Bonding
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Causes water to be a liquid rather than a gas at room
temperature
Holds enzymes in the shapes necessary for catalyzing biological
reactions
Causes strands of deoxyribonucleic acid (DNA) to pair up and
coil into a double helix
Noncovalent Interactions between Molecules
Hydrophilic (water-loving)
• Dissolves in water
• Table sugar
•
Has ionic charges, polar –OH groups, in its structure
Hydrophobic (water-fearing)
• Does not dissolve in water
• Vegetable oil
•
Does not have groups that form hydrogen bonds
John E. McMurry
http://www.cengage.com/chemistry/mcmurry
Chapter 3
Organic Compounds: Alkanes and
Their Stereochemistry
Problems: 3.1-24, 26-27, 29-30, 32, 34, 37-38, 40-48.
Richard Morrison • University of Georgia, Athens
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3.1 Functional Groups
Functional group
• An atom or a group of atoms that is part of a larger molecule
and that has a characteristic chemical reactivity
• Structural features that allow for classification of compounds
into families
Functional Groups
Functional Groups
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Functional Groups
Functional Groups
Functional Groups
Functional group
•
•
A given functional group behaves in nearly the same way in every
molecule it is a part of
The chemistry of every organic molecule, regardless of size and
complexity, is determined by the functional groups it contains
• In the reactions of ethylene and menthene with bromine both
molecules react with Br2 in the same way
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Functional Groups
Functional Groups with Carbon-Carbon Multiple Bonds
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Alkenes have double bonds
Alkynes have triple bonds
Arenes have alternating double and single bonds in a six-membered
carbon ring
The structural similarities in these compounds lead to
chemical similarities
Functional Groups
Functional Groups with Carbon Singly Bonded to an
Electronegative Atom
Example functional groups
include alkyl halides
(haloalkanes), alcohols,
ethers, alkyl phosphates,
amines, thiols, sulfides
and disulfides
Bonds are polar
Carbon bears a partial
positive charge (d+)
Electronegative atom
bears a partial negative
charge (d-)
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Functional Groups
Functional Groups with a Carbon-Oxygen Double Bond
(Carbonyl Groups)
• Carbonyl group
• C=O
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Carbonyl carbon bears a partial positive charge (d+)
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Carbonyl oxygen bears a partial negative charge (d-)
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Functional Groups
Functional Groups with a Carbon-Oxygen Double Bond
(Carbonyl Groups)
Present in organic compounds and in biological molecules
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Aldehydes
Ketones
Carboxylic acids
Esters
Thioesters
Amides
Acid chlorides
3.2 Alkanes and Alkane Isomers
Alkanes are the simplest family of molecules
Alkane
•
A compound of carbon and hydrogen that contains only carboncarbon single bonds from the s-overlap of sp3 hybrid orbitals
• General formula
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CnH2n+2 where n is an integer
Alkanes and Alkane Isomers
Alkanes often described as saturated hydrocarbons
• Hydrocarbons
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Contain only carbon and hydrogen
• Saturated
• Contain maximum possible number of hydrogens per carbon
and have only C-C and C-H single bonds
Alkanes occasionally referred to as aliphatic compounds, a name
derived from the Greek word aleiphas, meaning “fat”
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Alkanes and Alkane Isomers
•
Structures of representative alkanes
Alkanes and Alkane Isomers
Straight-chain alkanes or
normal alkanes
• Alkanes whose carbon
atoms are connected
without branching
Branched-chain alkanes
• Alkanes that contain a
branching connection of
carbons as opposed to a
straight-chain alkane
Alkanes and Alkane Isomers
Isomers
• From the Greek isos + meros
meaning “made of the same parts”
• Compounds that have the same
molecular formula but different
structures
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Alkanes and Alkane Isomers
Constitutional isomers
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Isomers that have their atoms connected in a different order
May have different carbon skeletons, different functional
groups, or different position of the functional groups
Alkanes and Alkane Isomers
• A given alkane can be drawn arbitrarily in many ways
• Representations of butane n-C4H10 where n denotes normal
(straight-chain)
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These representations indicate only the connections among
atoms
These representations do not imply any particular threedimensional geometry
Chemists rarely draw all the bonds in a molecule and usually
refer to the condensed structure
Alkanes and Alkane Isomers
• Names of straight-chain
alkanes
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Named according to the
number of carbon atoms in
the molecule, with the
exception of the first four
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Names based on Greek
numbers
Suffix –ane added to identify
the molecule as alkane
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Worked Example 3.1
Drawing the Structures of Isomers
Propose structures for two isomers with the formula
C2H7N.
3.3 Alkyl Groups
Alkyl group
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The partial structure that remains when a hydrogen atom is
removed from an alkane
• Not stable compounds
For naming of alkyl groups replace –ane with –yl ending
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Methane CH4
Ethane CH3CH3
methyl –CH3
ethyl –CH2CH3
Alkyl Groups
Combining alkyl groups with various functional groups
generates thousands of compounds
• Straight chains are generated by removing a hydrogen
from an end carbon
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Alkyl Groups
• Branched alkyl groups
are generated by
removing a hydrogen
atom from an internal
carbon
Alkyl Groups
Prefixes for C4 alkyl groups
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Refer to the number of other carbon atoms attached to the
branching carbon atom
• Four possibilities
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Primary (1º)
Secondary( 2º)
Tertiary (3º)
Quaternary (4º)
R represents a generalized organic group
Alkyl Groups
The terms primary, secondary, tertiary, and quaternary
are routinely used to identify structural features
• Citric acid is a tertiary alcohol
• Citric acid has an alcohol functional group –OH bonded to a
carbon atom that is itself bonded to three other carbon atoms
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Alkyl Groups
Hydrogen atoms
• Primary
• Attached to primary carbons (RCH3)
• Secondary
• Attached to secondary carbons (R2CH2)
• Tertiary
• Attached to tertiary carbons (R3CH)
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