block 1 - foundations Flashcards
what makes sigma and pi bonds
sigma = end on overlap of atomic orbitals --> stronger pi = side on overlap of atomic orbitals --> weaker
number of pi and sigma bonds in single, double and triple bonds
single = sigma double = 1 sigma 1 pi triple = 1 sigma 2 pi
sp3, sp2 sp hybridisation
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
constitutional isomers
same molecular formula, diff atom to atom bonding
no. of constitutional isomers increases with size
double bond equivalents (constitutional isomers)
no. of rings or pi bonds = 1/2 (2n4 + n3 - n1 +2)
n4 = carbon, n3 = nitrogen, n1 = hydrogen/halogen
how to systematically draw constitutional isomers
with each category, change one dimension at a time
reduce ring size (if ring) systematically & explore substituent position around the ring
incorporate functional group
conformational isomers
same molecular formula, same atom to atom bonding sequence but diff arrangement in space; rotation around single bond
are same compound
types of conformers
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
types of cyclohexane conformers
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
ring flip ( cyclohexane conformers)
rotating chair conformers (flip down side up, up side down) axial groups become equatorial groups vice versa but UP & DOWN DON’T CHANGE
substituted cyclohexane conformers stability
favoured conformer = largest atom/group in EQUATORIAL position (minimised repulsion)
not possible to have all substituents eq. –> largest group eq. = favoured
configurational isomers
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
cis/trans isomers (cyclic compounds - configurational isomers)
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
CIP rules (e/z isomers)
- higher atomic number = higher priority
- if 2 directly attached atoms same –> one step out until first difference
- if group attached has double bond, treat as 2 single bonds to same element
- high priority on SAME side = Z; high priority on DIFFERENT sides = E
enantiomer rotation
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
R/S notation for enantiomers
R = priority groups ranked CLOCKWISE S = ANTICLOCKWISE
diasteriomers
configurational isomers that don’t have a mirror image relationship
can occur when there are multiple stereo centres in the same molecule
electronegativity and polarity
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
intermolecular forces
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
what is boiling point of a compound related to
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
polar bonds, solubility and chromatography
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
substitution, addition, elimination
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
polar reactions
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)
homolytic vs heterolytic bond cleavage
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
change in formal charge when going from reaction to product
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
carbocations (relative stability, geometry)
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
nucleophilic substitution - SN2
reagent = nucleophile
occurs in a single step - bond breaking/forming is simultaneous
nucleophilic substitution - SN1
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
reactions of alkenes and alkynes
electrophilic addition
alkene hydrogenation
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
H+ as electrophile in alkenes
addition of HZ (Z = OH, halogen, OR) via carbocation intermediate
- addition of H+ to C=C –> carbocation (lose e- in pi bond w C –> C+ only has 3 bonds)
- addition of nucleophile to carbocation - nucleophile forms bond to C+
markovnikovs rule
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)
alkene halogenation
- halogen acts as electrophile (break C=C bond, carbocation intermediate (bonded to 1 X) + X-
- 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
benzene, resonance energy
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
naming benzene derivatives
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
alkyne hydrogenation
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)
alkyne + HX and X2
markovnikovs rule APPLIES
can be stopped after addition of ONE mole equivalent –> ALKENE (therefore excess –> ALKANE)
ANTI stereochemistry of addition
alkyne hydration
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
acid/base reactions of alkynes
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
