Transcript Document 7552323

Molecular Structure and Organic
Chemistry
• The structure of a molecule refers to the
arrangement of atoms within the molecule. The
structure of a molecule is critical to the chemical and
physical properties of a substance. In fact so vital is
structure to molecular identity that the same
molecular formula may represent more than one
substance based upon their differing structures.
• Example: Two totally different substances share
the same molecular formula, C2H6O, but are
different because of their differing molecular
structures.
• Ethyl Alcohol C2H6O Dimethyl Ether C2H6O
Importance of Molecular Structure
• Ethyl Alcohol C2H6O
CH3CH2
O
H
Dimethyl Ether C2H6O
CH3
O
CH3
Isomers
• Molecules that have the same molecular
formulas but different structures are called
isomers
• There are 2 isomers corresponding to the
molecular formula C4H10 , 3 corresponding to
C5H12 and 39 corresponding to the molecular
formula C9H20
CH3CH2CH2CH3
CH3CH2CH2CH2CH3
CH(CH3)3
CH3CH2CH(CH3)2
C(CH3)4
•
•
•
using molecular vibrations as a key to structure
Although ball and stick models of molecules are
very effective at approximating the actual shapes
of molecules, they do have one major flaw they
leave you with the false impression that
molecules are rigid objects
The atoms that make up molecules are in
constant motion. They are continually flexing
about their bonds. This flexing is referred to as
vibrational motion. There are several types of
vibrations, the two most basic vibrational modes
are stretching and bending.
Some Vibrational Modes
Show clip of various vibrational modes here.
=>
Molecular Vibrations
Covalent bonds vibrate at only certain
allowable frequencies.
=>
Vibrations as a Key to Structure
• Each type of vibration has a frequency that
depends upon the:
• For a constant bond type (single, double or
triple) the frequency of the vibration is low for a
bond between heavy atoms. Conversely, for a
given bond type the frequency of vibrations is
high for light atoms.
• Multiple bonds vibrate at a higher frequency
than do single bonds
Stretching Frequencies
• Frequency decreases with increasing
atomic weight.
• Frequency increases with increasing
bond energy.
=>
Infrared Absorbtions
• Vibrations as a key to structure- the entire range
of vibrations for all organic molecules falls within
the Infrared Region of the Electromagnetic
Spectrum(2500nm – 25000nm). If a beam of IR
radiation is directed at a molecular sample and if
the beam has the same frequency as one of the
vibrational modes of the molecule then the
molecule will absorb the energy of the IR
radiation and the molecular vibration will
increase in intensity.
The IR Region
• Just below red in the visible region.
• Wavelengths usually 2.5-25 mm.
• More common units are wavenumbers, or
cm-1, the reciprocal of the wavelength in
centimeters.
• Wavenumbers are proportional to
frequency and energy.
=>
Infrared Absorbtions
• If in order for absorption to occur, the IR frequency must
match the frequency of the vibrating atoms, and if the
frequency of the vibrating atoms is dependent upon the
mass of the atoms and the bond type; then the
frequency at which absorbance occurs is dependant
upon the mass of the atoms and the bond type.
Therefore, the same two bonded atoms, regardless of
the molecule that they are in, will have the same
absorbance frequency in the IR region. This is the major
strength of IR Spectrophotometry. IR Spec identifies the
•
present in an organic molecule.
• Functional Group- this is an atom or group of
atoms that imparts a unique set of chemistry to
whatever organic molecule it is bonded to. If the
same functional group is attached to two
different organic molecules then the two organic
molecules will have similar chemistry’s and have
similar absorbance values of the IR Specs.
• Show clip of absorbtion frequencies for various
functional groups.
O-H and N-H Stretching
• Both of these occur around 3300 cm-1, but
they look different.
– Alcohol O-H, broad with rounded tip.
– Secondary amine (R2NH), broad with one
sharp spike.
– Primary amine (RNH2), broad with two sharp
spikes.
– No signal for a tertiary amine (R3N)
=>
An Alcohol IR Spectrum
=>
An Amine
IR Spectrum
=>
Carbonyl Stretching
• The C=O bond of simple ketones,
aldehydes, and carboxylic acids absorb
around 1710 cm-1.
• Usually, it’s the strongest IR signal.
• Carboxylic acids will have O-H also.
• Aldehydes have two C-H signals around
2700 and 2800 cm-1.
=>
A Ketone
IR Spectrum
=>
An Aldehyde
IR Spectrum
=>
O-H Stretch of a
Carboxylic Acid
This O-H absorbs broadly, 2500-3500 cm-1,
due to strong hydrogen bonding.
=>
Variations in
C=O Absorption
• Conjugation of C=O with C=C lowers the
stretching frequency to ~1680 cm-1.
• The C=O group of an amide absorbs at an
even lower frequency, 1640-1680 cm-1.
• The C=O of an ester absorbs at a higher
frequency, ~1730-1740 cm-1.
• Carbonyl groups in small rings (5 C’s or
less) absorb at an even higher frequency. =>
An Amide
IR Spectrum
=>
Carbon - Nitrogen
Stretching
• C - N absorbs around 1200 cm-1.
• C = N absorbs around 1660 cm-1 and is
much stronger than the C = C
absorption in the same region.
• C  N absorbs strongly just above 2200
cm-1. The alkyne C  C signal is much
weaker and is just below 2200 cm-1 .
=>
A Nitrile
IR Spectrum
=>
IR Spectrophotometers
• Because every molecule has a unique set of
atoms and bonds that compose it, each
molecule will absorb IR radiation only at certain
frequencies. These frequencies are related to
the types of bonds and arrangements of atoms
in a molecule.
• An IR Spectrophotometer is an instrument that
measures the absorbance of IR radiation by a
sample as a function of frequency.
The Hydrocarbon Skeleton
• All organic molecules consist of nothing more
than a hydrocarbon skeleton with functional
groups hung off that skeleton at various
positions.
• We have just seen that functional groups can
be identified by their characteristic IR
absorbtions.
• Let us now investigate the characteristic IR
absorbtions of the hydrocarbon skeleton
Carbon-Carbon
Bond Stretching
• Stronger bonds absorb at higher
frequencies:
– C-C
– C=C
– CC
1200 cm-1
1660 cm-1
2200 cm-1 (weak or absent if internal)
• Conjugation lowers the frequency:
– isolated C=C
1640-1680 cm-1
– conjugated C=C 1620-1640 cm-1
– aromatic C=C
approx. 1600 cm-1
=>
Carbon-Hydrogen Stretching
Bonds with more s character absorb at a
higher frequency.
– sp3 C-H, just below 3000 cm-1 (to the right)
– sp2 C-H, just above 3000 cm-1 (to the left)
– sp C-H, at 3300 cm-1
=>
An Alkane IR Spectrum
=>
An Alkene IR Spectrum
=>
An Alkyne IR Spectrum
=>
Vibrational Motions