block 1 - foundations

Question 1

what makes sigma and pi bonds

Answer 1

sigma = end on overlap of atomic orbitals --> stronger
pi = side on overlap of atomic orbitals --> weaker

Question 2

number of pi and sigma bonds in single, double and triple bonds

Answer 2

single = sigma
double = 1 sigma 1 pi
triple = 1 sigma 2 pi

Question 3

sp3, sp2 sp hybridisation

Answer 3

sp3
- alkanes bonded to 4 atoms
tetrahedral 109º
- 2s orbital mix w/ 3 2p orbital
- 4 sigma bonds
sp2
- alkenes bonded to 3 atoms trigonal planar 120º
- 2s orbital mix w/ 2 2p orbitals –> 1p orbital unhybridised –> 1 pi bond, 3 sigma bonds
sp
- alkynes, bonded to 2 atoms linear 180º
- 2s orbital mix w/ 1 2p orbital –> 2 p unhybridised –> 2 pi bonds, 2 sigma bonds

Question 4

constitutional isomers

Answer 4

same molecular formula, diff atom to atom bonding

no. of constitutional isomers increases with size

Question 5

double bond equivalents (constitutional isomers)

Answer 5

no. of rings or pi bonds = 1/2 (2n4 + n3 - n1 +2)

n4 = carbon, n3 = nitrogen, n1 = hydrogen/halogen

Question 6

how to systematically draw constitutional isomers

Answer 6

with each category, change one dimension at a time
reduce ring size (if ring) systematically & explore substituent position around the ring
incorporate functional group

Question 7

conformational isomers

Answer 7

same molecular formula, same atom to atom bonding sequence but diff arrangement in space; rotation around single bond
are same compound

Question 8

types of conformers

Answer 8

ANTI - staggered, largest groups furthest apart –> minimised repulsion –> more stable, low energy, most time spent
GAUCHE - staggered, largest groups not furthest apart, more repulsion than anti but less that syn
SYN - eclipsed, largest groups eclipsed –> least stable, high energy

Question 9

types of cyclohexane conformers

Answer 9

CHAIR - 2 sets of hydrogens alternating above and below ring. axial = up or down, equatorial = horizontal. each c with 1 A 1 E hydrogen. bonds not eclipsing = lowest energy, most stable
BOAT - eclipsing = highest energy, least stable

Question 10

ring flip ( cyclohexane conformers)

Answer 10

rotating chair conformers (flip down side up, up side down) axial groups become equatorial groups vice versa but UP & DOWN DON’T CHANGE

Question 11

substituted cyclohexane conformers stability

Answer 11

favoured conformer = largest atom/group in EQUATORIAL position (minimised repulsion)
not possible to have all substituents eq. –> largest group eq. = favoured

Question 12

configurational isomers

Answer 12

same molecular formula, same atom to atom bonding, different arrangement in space - converted by breaking and reforming covalent bonds
enantiomers = non-superimposable mirror images
diastereomers = everything else

Question 13

cis/trans isomers (cyclic compounds - configurational isomers)

Answer 13

no rotation in rings –> substituents only above or below plane of ring, fixed
cis = same side
trans = opposite sides
cis & trans relative terms - explain where one sub. is in relation to other

Question 14

CIP rules (e/z isomers)

Answer 14

1. higher atomic number = higher priority
2. if 2 directly attached atoms same –> one step out until first difference
3. if group attached has double bond, treat as 2 single bonds to same element
4. high priority on SAME side = Z; high priority on DIFFERENT sides = E

Question 15

enantiomer rotation

Answer 15

interact with plane polarised light –> optically active
amount of rotation is characteristic of enantiomer
mixture in equal amounts of two enantiomers = racemic mixture - rotate light equally in both directions –> overall rotation of 0

Question 16

R/S notation for enantiomers

Answer 16

R = priority groups ranked CLOCKWISE
S = ANTICLOCKWISE

Question 17

diasteriomers

Answer 17

configurational isomers that don’t have a mirror image relationship
can occur when there are multiple stereo centres in the same molecule

Question 18

electronegativity and polarity

Answer 18

a measure of the ability of a bonded atom to attract electrons to itself
difference in en = increases polarity
more en = partial negative charge vice versa
double bonds more readily polarised - relative mobility of pi e-. more polar than single bonds

Question 19

intermolecular forces

Answer 19

MOMENTARY DIPOLE DIPOLE
e- mobile –> not evenly distributed –> temporary dipole (one side more negative than other) –> distortions in neighbouring e- clouds –> attractive electrostatic forces between molecules
larger & more polarisable e- cloud –> stronger mdd interactions
increases with size
weakest but DOMINANT
DIPOLE DIPOLE
polar bonds –> e- not symmetrical –> permanent dipole
increase with overall polarity
HYDROGEN BONDS
dipole dipole - O-H, F-H, N-H = very polar

Question 20

what is boiling point of a compound related to

Answer 20

MOLECULAR SIZE - increases with molecular size (momentary dipole dipole)
MOLECULAR SHAPE - linear = stronger interactions, more contact so higher boiling point. intermolecular forces strongest when molecules in close proximity –> decreased branching = increased bp
PRESENCE OF POLAR BONDS
for compounds w similar molecular weights, MORE polar functional groups = higher bp (increased dipole dipole)
for molecules w different sizes - difference in momentary dd = biggest effect on bp

Question 21

polar bonds, solubility and chromatography

Answer 21

more polar = more water soluble (hydrophilic)
less polar = more lipid soluble (lilophylic)
chromatography - polar stationary phase (onto which mixture of interest absorbed) and polar mobile phase (solvent)
polar bonds attracted to polar stationary phase –> move slower
non-polar bonds less attracted to polar stationary phase –> move faster

Question 22

substitution, addition, elimination

Answer 22

substitution = sigma bond broken, sigma bond formed. same dbe
addition = pi bond broken, 2 sigma bonds formed. dbe decrease
elimination = 2 sigma bonds broken, 1 pi bond formed. dbe increase

Question 23

polar reactions

Answer 23

when nucleophile and electrophile react to form a new covalent bond
nucleophile = electron donor, electron rich, neutral or negatively charged. e- in pi bonds, -ve in polar bond, lone pairs (neutral/neg charged)
electrophile = electron deficient. neutral or positively charged (+ve of polar bond)

Question 24

homolytic vs heterolytic bond cleavage

Answer 24

homolytic = one electron from bond on each atom that was formerly bonded --> free radical
heterolytic = both electrons from bond end up on one of the atoms formerly bonded. e- move to the MORE en atom

Question 25

change in formal charge when going from reaction to product

Answer 25

A LOSES ownership over one of (previously) shared e- and becomes a unit of charge more POSITIVE
B GAINS ownership over one of (previously) shared e- and becomes a unit of charge more NEGATIVE

Question 26

carbocations (relative stability, geometry)

Answer 26

carbocations more common than carbanions (C not very en)
3º > 2º > 1º > methyl
planar (sp2 hybridised)
localised charge = less stable so that’s why 3º more stable

Question 27

nucleophilic substitution - SN2

Answer 27

reagent = nucleophile

occurs in a single step - bond breaking/forming is simultaneous

Question 28

nucleophilic substitution - SN1

Answer 28

reagent = nucleophile
multiple steps - bond breaking –> bond forming
1. heterolytic bond breaking –> carbocation intermediate
2. polar bond formation w carbocation as electrophile
3. heterolytic bond breaking to give neutral product

Question 29

reactions of alkenes and alkynes

Answer 29

electrophilic addition

Question 30

alkene hydrogenation

Answer 30

reagent = H2/catalyst (eg. Pt)
sometimes called reduction
occurs with SYN stereochemistry (same face of double bond)
H bonds, same time other groups pushed away in same direction

Question 31

H+ as electrophile in alkenes

Answer 31

addition of HZ (Z = OH, halogen, OR) via carbocation intermediate

1. addition of H+ to C=C –> carbocation (lose e- in pi bond w C –> C+ only has 3 bonds)
2. addition of nucleophile to carbocation - nucleophile forms bond to C+

Question 32

markovnikovs rule

Answer 32

addition of an unsymmetrical reagent to an unsymmetrical alkene, major and minor products
major = more substituted product
carbocation C bonded to more c (3º most stable –> more readily formed –> more product from that cation - major)

Question 33

alkene halogenation

Answer 33

1. halogen acts as electrophile (break C=C bond, carbocation intermediate (bonded to 1 X) + X-
2. halide ion acts as nucleophile - X- forms bond with C+ from carbocation
   occurs with ANTI stereochemistry
   halogen intermediate where one of X has 2 bonds (so X+) to C and blocks bottom face –> other X can’t face bottom so faces top

Question 34

benzene, resonance energy

Answer 34

6 carbon ring, all C are sp2 hybridised
6 identical ‘1.5’ bonds
RE = conjugated double bonds give benzene extra stability –> react in a different way to regular alkenes: NO ELECTROPHILIC ADDITION

Question 35

naming benzene derivatives

Answer 35

amine group = aniline
aldehyde group = benzaldehyde
carb acid group = benzoic acid
alcohol = phenol
methyl = toluene
for simple derivatives - suffice benzene used
complex molecules = phenyl branch (Ph)
DISUBSTITUTED BENZENES:
1,2 = ortho
1,3 = meta
1,4 = para

Question 36

alkyne hydrogenation

Answer 36

depends on number of mol equivalents , and required stereochemistry
LINDLARS CATALYST/H2 –> Z ALKENE
Li/LIQ. NH3 (1) , H2O (2) –> E ALKENE
Pt OR Pd / H2 –> ALKANE (electrophilic addition twice)

Question 37

alkyne + HX and X2

Answer 37

markovnikovs rule APPLIES
can be stopped after addition of ONE mole equivalent –> ALKENE (therefore excess –> ALKANE)
ANTI stereochemistry of addition

Question 38

alkyne hydration

Answer 38

H2SO4/HGSO4 (addition of water)
addition of only one mol eq. of water occurs
for all alkynes except ethyne (p= aldehyde) , products are KETONES
tautomeric equilibrium, species involved = tautomers

Question 39

acid/base reactions of alkynes

Answer 39

formation of alkynide anions from terminal (c triple bonded to c and also H - end of molecule) alkynes
H on terminal alkyne = weakly acidic –> removed by strong base (eg. NH2)
carbANION formed as intermediate (c = nucleophile) - useful in synthesis for carbon chain extension