CHEM 217 Lecture 10: Unit 10
Unit 10
1 0 . 1 A R T I F I C I A L S W E E T E N E R S
Taste of food independent of its metabolism (some artificials don’t get metabolized)
Tongue discerns taste- main facrots for this discrimination is molecule’s shape and charge distribution
Surface of taste cell has proteins that are taste receptors – particular tastants fit snugly on the active site
Both aspartame and saccharin bind into active site in t1r protein more strongly than sugar
1 0 . 2 V S E P R B A S I C S H A P E S
Electron groups (define as long pairs, single bonds, multiple bonds and even single electron) repel one
another through coulombic forces (forces on repulsions)
According to VESPR theory, repulsions between electron groups on interior atoms of a molecule
determine geometry – preferred geometry is max separation between electrons (i.e minimum energy)
oFor molecules having one interior atom (the central atom) molecular geometry depends on
Number of electron groups
Howf many groups are bonding and lone pairs
TWO ELECTRON GROUPS: LINEAR GEOMETRY
With two electron groups (two single bonds – rare because no octet) maximize separation by assuming
180° bond angle or a linera geometry
Same is for any molecule with two electron groups and no lone pairs
THREE ELECTON GROUPS: TRIGONAL PLANAR GEOMETRY
Three electron groups can maximize separation by assuming 120° bond angle in a plane, a trigonal planar
geometry. What if two single bonds and one double bond? Angles are ~120° and not exact. VSEPR
good predictor but doesn’t account for tiny differences.
FOUR ELECTRON GROUPS
Molecules with four electron groups around the central atom, geometry is 3D. Tetrahedral shape produces
109.5° bond angles between balloons.
Geometrical shape with 4 identified faces, each on an equilateral triangle
FIVE ELECTRON GROUPS: TRIGONAL BIPYRAMIDAL GEOMETRY
Three groups lie in a single plane while other two above and below this plane
Angles in by pyramidal structure not all the same
Angles between the equatorial positions (3 bonds in the trigonal plane) are 120° while angle between axial
positions and the trigonal plane is 90°
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SIX ELECTRON GROUPS: OCTAHEDRAL GEOMETRY
4 of the groups lie in a single plane with a fifth group above the plane and another below it. All angles are
90°
1 0 . 3 V S E P R T H E O R Y A N D T H E E F F E C T S O F L O N E P A I R S
Lone pairs also repel
If we don’t distinguish between bonding electrons and lone pairs, we find the electron geometry
(arrangement of electron groups)
The molecular geometry arrangement of atoms and electron geometry are different, but electron
geometry is relevant to molecular geometry
Different kinds of electron groups result in different amounts of repulsion
Lone pair electron groups generally exert slightly greater repulsions – they occupy more angular space
because they’re closer to the nucleus and spread out between two atoms
Two lone pairs and two bonding: electron geometry also tetrahedral but molecular gemotry is bent. Bond
angles would be slightly smaller than ideal tetrahedral because of greater repulsion by two lone pairs
MOST REPULSIVE: Lone-lone > Lone-bonding > bonding-bonding : LEAST REPULSIVE
FIVE ELECTRON GROUPS WITH LONE PAIRS
When there are 4 bonding and 1 lone pair – notice lone pair could occupy either equatorial or axial
position
oLone pair should occupy position that minimizes its interaction with bonding pairs
In an axial position it would have three 90° interactinos with bonding pairs
In an equatorial position it has two 90° interactions with bonding pairs
The lone pair occupies an equatorial position
oThe resulting molecular geometry is a seesaw
When two of five electrons are lone pairs, they occupy 2/3 of the equatorial positions and
minimize 90° interactions with bonding pairs and also avoiding a lone-pair-lone-pair repulsion
Resulting geometry is T-shaped
When 3/5 electron groups around a central atom are lone, the lone occupy all equatorial positions
and resulting geometry is linear
5 ELECTRON GROUPS WITH LONE PAIRS
Octahedral. Since all six positions are equivalent, the lone pair can be situated on any of these positions –
resulting geometry is square pyramidal
When two of the six electron groups are lone, lone pairs occupy positions across rom one another-
resulting molecular geometry is square planar
VSEPR SUMMARY
Geometry is determined by # of electron groups on central atoms (or on all interior atoms if 1+)
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# of electron groups can be determined from lewis structure. If there is a resonance structure, use any one
of them to determine # of electron groups
Each of these count as a group
Lone pair, single bond, double bond, triple bond, single electron
Geometry determined by repulsions
Bond angles vary from idealized angles because multiple bonds occupy more space (bulkier) and lone pairs
occupy more space
PREDICTING SHAPES OF LARGER MOLECULES
Many have 2+ interior atoms
When predicting, principles covered must apply to each interior atom, must determine geometry of each
1 0 . 5 S H A P E A N D P O L A R I T Y
If a diatomic molecule has a polar bond, the molecule as a whole will be polar and vice versa
In polyatomic molecules, presence of polar bonds may or may not result in polar molecules
If the molecular geometry is such that dipole moments of individual bonds sum to a net dipole moment
then it is polar
oIf the dipole moments of individual polar bonds cancel each other (sum zero) then nonpolar
Dipole moments can cancel each other out because they are vector quantities (they have both a
magnitude and a direction)
oThink of each polar bond as a vector pointing in a direction of the more electronegative atom
oLength of the vector proportional to the electronegativity difference between bonding atoms
In linear, dipole moments easily cancel if there is a change of shape that can change.
Summary:
Shape polarity
oDraw lewis structure and determine molecular geometry
Determine if there are polar bonds
Bond is polar if the two bonding atoms have sufficiently different
electronegativities
If it has polar bonds, superimpose vector on each bond, making length
proportional to electronegativity difference between bonding atoms
Determine if polar bonds add together to form a net dipole moment.
oSum vectors corresponding to polar-bonds together. If the vector
sums zero then the molecule is nonpolar.
oIf there is a net sum, the molecule is polar
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
1 a r t i f i c i a l s w e e t e n e r s. Taste of food independent of its metabolism (some artificials don"t get metabolized) Tongue discerns taste- main facrots for this discrimination is molecule"s shape and charge distribution. Surface of taste cell has proteins that are taste receptors particular tastants fit snugly on the active site. Both aspartame and saccharin bind into active site in t1r protein more strongly than sugar. 2 v s e p r b a s i c s h a p e s. Electron groups (define as long pairs, single bonds, multiple bonds and even single electron) repel one another through coulombic forces (forces on repulsions) According to vespr theory, repulsions between electron groups on interior atoms of a molecule determine geometry preferred geometry is max separation between electrons (i. e minimum energy) o. For molecules having one interior atom (the central atom) molecular geometry depends on.
