block 1 - foundations Flashcards
(39 cards)
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
