Lectures 9-21 Flashcards
drawing Lewis Structures
sum valence electrons, place single bond between atoms, complete octet rules, add lone pairs/multiple bonds, place extra electrons in d orbitals if necessary
the more resonance structures a molecule has,
the more stable it is
bond order
0.5 x (number of delocalised electrons/number of bonds containing delocalised electrons)
Lewis Acids
can accept a pair of unbonding electrons
all species with an odd number of electrons are
radicals
number of electron pairs: linear
2
number of electron pairs: trigonal planar
3
number of electron pairs: tetrahedral
4
number of electron pairs: trigonal bipyramidal
5
number of electron pairs: octahedral
6
linear bond angle
180
trigonal planar bond angle
120
tetrahedral bond angle
109.5
trigonal bipyramidal bond angle
90, 120 and 180
octahedral bond angle
90 and 180
lone pairs occupy
more space than bonding pairs
2 lone pairs, 2 bonds
bent
3 bond pairs, 1 lone pair
trigonal-pyramidal
2 bond pairs, 1 lone pair
V-shaped
Lewis Structures are useful for
electron counting and description of bonding
molecular shape considers that lone pairs
push actual atoms in certain ways
trigonal bipyramidal with one lone pair molecular shape
see-saw
octahedral with one lone pair molecular shape
square pyramid
octahedral with two lone pairs molecular shape
square planar
Molecular Orbital
orbital overlap
sigma bond
covalent bond for two electrons; elongated, ellipse shape
when a bond forms, energy is
released
sp3 hybrid orbitals are
tetrahedral
C-C bond is weaker than
C-H bond
structural isomers differ in
connectivity
isopropyl
-CH(CH3)2
tert-Butyl
-C(CH3)3
average bond order
bonds / # resonance structures
double and triple bonds in geometry only count as
one electron pair
formal charge (where valence electrons means from original atom)
valence electrons - # lone pair electrons - 1/2(# bonding electrons)
we number chain closest to
branch point
conformational isomers
differ due to bond rotation
bond rotation slows with
cooling
Newman Projection
looking down with carbons in a row
dihedral angle
angle between front and back hydrogen/group when looking at a molecule via Newman Projection
staggered
most stable conformation; hydrogens/groups as far apart as possible
eclipsed
least stable conformation; increase in potential energy
torsional strain
bonds-electron repulsion
in eclipsed conformation, there is more
torsional strain
antiperiplanar staggered
`most stable for butane; two methyl groups as far apart from each other as possible
gauche staggered
methyl groups at 60 degrees; steric strain
anticlinal eclipsed
eclipsed but methyl groups not close for butane
synperiplanar eclipsed
methyl groups as close as they can be to each other; steric and torsional strain
bigger substituents near each other,
higher energy (gauche/eclipsed)
to calculate conformer energy values,
add up gauche/eclipsed values between ALL groups (use table)
chiral
molecules that have a non-superimposable mirror image
stereoisomers
same numbers and types of atoms, differ in three-dimensional space
asymmetric carbon atom
stereogenic centre
enantiomers
pairs of non-superimposable mirror image molecules
racemate
1:1 mixture of enantiomers
in a 1:1 mixture of enantiomers, rotation of plane polarised light would be
zero
enantiomers rotate plane polarised light in
opposite directions
optically active
a compound that rotates plane polarised light
enantiomers have the same physical and chemical characteristics except
in their behaviour towards plane polarised light and their reactivity in a chiral environment
specific rotation [alpha]D
alpha/l x c
assigning absolute configuration about an asymmetric carbon
priority based on decreasing atomic number, viewing molecule along bond from asymmetric carbon to lowest priority
priority #1 atomic number is at
bottom
if clockwise, denoted as
R
if anticlockwise, denoted as
S
phantom atoms
if a carbon is double bonded to an atom, we treat it as a single bond to two of this atom
the more H atoms a carbon is bonded to,
the lower in priority
number of stereoisomers
2^n, where n is number of asymmetric centres
mirror images will have reversed
S and R
diastereoisomers
stereoisomers that are not enantiomers
S cancels
R
meso compound
stereoisomer with two or more asymmetric carbon atoms that is NOT chiral
ring strain
angle strain, torsional strain, steric strain
highest ring strain energy
cyclopropane
lowest ring strain energy
cyclohexane
preferred cyclohexane conformation
chair
all bonds in chair conformation are
staggered, so torsional strain is eliminated
axial hydrogens
parallel to axis that goes through centre of ring vertically
equatorial hydrogens
radiate out from equator of ring
rings on cyclohexane should be in
equatorial position
why should rings in cyclohexane be equatorial?
1,3-diaxial steric interaction
groups on same face of cycloalkane
cis
groups on different face of cycloalkane
trans
groups remain cis or trans even when they
flip between axial and equatorial
bigger group should be
equatorial
ax-eq and eq-ax are identical if
the substituents are the same
cis is higher in energy for fused cyclohexanes because
one carbon is axial
you cannot ring flip
trans-decalin cyclohexane
the more stable/conformationally rigid fused cyclohexane
trans-decalin
hybridised orbital for trigonal planar/double bonds
sp^2
hybridised orbital for tetrahedral
sp^3
pi bond
bond between two carbon atoms where we’ve formed a sigma bond and we’re overlapping two p orbitals side-on
pi region is
electron-rich
in the pi region, electrons are not as
tightly held
sigma bond is stronger than
pi bond
no rotation about a
pi bond
geometric isomers form due to to
inability of double carbon-carbon bonds to rotate
we number chain giving the double/triple bond the
lowest number/top priority
Z
of substituents of higher priority are on the same side of the double bond
E
if substituents of higher priority are on different sides of the double bond
which types of alkenes are less stable, cis or trans?
cis
steric strain is present in
cis alkenes
cumulated/allenes
consecutive double bonds
conjugated
double-single-double
isolated
double-single-single-single-etc.-double
bonds with partial double bond character will be
shorter
partial double bond character is present in
conjugated systems
conjugated systems are
more stable
benzene is comprised of a
continuous pi cloud circuit with partial double bond character throughout the entire loop
resonance stabilisation
electrons are delocalised in pi cloud
ortho/o
1,2
meta/m
1,3
para/p
1,4
naming benzenes with more than two substituents
number substituents to give lowest possible numbers, list alphabetically
when do we number benzenes according to the chain
if the chain attached to the benzene has more than six carbons
constitutional isomer
same molecular formula, different structure
double bonds are composed of
one sigma bond and one pi bond
all atoms directly attached to double bond will be
coplanar
aromatic compounds
planar, cyclic, conjugated, Huckel’s rule
Huckel’s rule
4n + 2 pi electrons where n is a whole number
sp hybridised
linear, triple bonds
each sp bond contains
two pi bonds and one sigma bond
primary carbon
carbon bonded to one carbon
secondary carbon
carbon bonded to two carbons
tertiary carbon
carbon bonded to three carbons
quaternary carbon
carbon bonded to four carbon
benzylic carbon
carbon directly attached to a carbon of a benzene ring (sp^3)
aryl carbon
carbon part of benzene ring (sp^2)
vinylic carbon
carbon that is part of a double bond (sp^2)
allylic carbon
carbon bonded to a carbon-carbon double bond carbon (bonded to a vinylic carbon, sp^3)
primary alcohol
hydroxyl carbon bonded to one other carbon
secondary alcohol
hydroxyl carbon bonded to two other carbons
tertiary alcohol
hydroxyl carbon bonded to three other atoms
ether
oxygen atom bonded to two carbons
ether nomenclature
add ether on the end with a space; alkyl groups are substituents
thiol
sulphur bonded to carbon and hydrogen
thiol nomenclature
add ‘thiol’ on end
primary amine
N attached to one carbon
secondary amine
N attached to two carbons
tertiary amine
N attached to three carbons
alcohol has priority over
amine group
nitrogen has a lone pair which can be
donated in acid-base reactions
C=O bond
sigma C-O bond, pi C-O bond
carboxylic acids have priority over
alcohols and amines
primary amide
N is attached to two hydrogens and C=O
secondary amide
N is attached to one H, one R group and the C=O
tertiary amide
C=O is attached to two R groups and the C=O
nitriles
carbon-nitrogen triple bonds
polar bonds arise from
electronegativity differences
in polar bonds, partial positive charge is equal in magnitude to
partial negative charge
dipole moment
degree of polarity
dipole moment units
Debye (D)
dipole moment points towards
negative end of whole molecule
where there is symmetry/the negative ends cancel out, there will be
no net dipole moment
cis and trans isomers can have different
polarities
in the C=O atom, the pi bond/cloud is distorted towards the
O atom
dispersion forces
due to instantaneous/temporary dipole forces
attraction in dispersion forces is between
positive nucleus and negative electron cloud
dispersion forces increase with
size of atom
states of non-polar electron can be understood by the strength of
dispersion forces
alkanes have strong
dispersion forces
more branched molecules have weaker
dispersion forces
lone pairs contribute to hydrogen bonding by providing
localised centres of negative charge
if the oxygen is not attached to a hydrogen, it
cannot engage in hydrogen bonding
