Organic Chemistry Flashcards
how do hybrid orbitals form?
when s and p orbitals are close in energy they merge together to form hybrid orbitals
What are sp3 orbitals?
when 1s joins with three 2p orbitals, this is tetrahedral
what is sp2?
When 1s joins with two 2p orbitals, this is planar
What are sigma bonds?
sigma bonds are bonds formed between hybridised orbitals
What are pi bonds?
they are bonds formed between unhybridised 2p orbitals in the same plane (different plane to the sigma bonds)
What bonds are in double/triple bonds?
double = 1 sigma, 1 pi triple = 1 sigma, 2 pi
Describe orbitals and bonding in benzene
- Each carbon has four e-
- Each carbon is sp2 hybridised, each form 3 sigma bonds
- each carbon has an extra p orbital, all of which form a pi system
Describe the properties of an aromatic compound
- Must be cyclic
- Have a pi system (each atom must have an unhybridised p)
- Must be planar
- must have 4n+2 pi electrons, where n is an integer
What happens when orbitals combine in phase?
form bonding bond (orbital spread over both atoms)
what happens when orbitals combine out of phase?
form anti-bonding bond (node between 2 phases where no electrons can exist)
anti-bonding and bonding occupy the same space
but electrons fill the lowest energy orbitals and so go into bonding orbitals
Rules of molecular orbitals?
- No of Atomic orbitals = No of molecular orbitals
- In phase AOs = bonding
- out of phase AOs = anti-bonding
- AOs overlap to form MOs
- for efficient overlap, difference in AOs energy must be small
- bond order = 0 means bond doesnt exist
Equation for bond order?
bond order = (# e- in bonding - # e- in antibonding)/2
What is meant by conjugated?
When 2 double bonds are separated by a single bond, the p orbitals overlap to form a pi system (must be planar for a pi system)
Why is benzene more stable?
the energy of bonding in benzenes MOs is less after conjugation making it more stable
Equation to find difference in energy between homo and lumo? What happens when the E is of the right wavelength?
Delta E = hv = hc/lamda
- energy of right wavelength = e- excited
What does it mean when a molecular is more conjugated?
more conjugated = smaller energy gap = larger wavelength
What do HOMO and LUMO stand for?
Highest occupied molecular orbital
Lowest unoccupied molecular orbital
why is the homo lumo gap important?
This is the smallest gap so it is where electrons are most likely to be excited
What does a smaller homo lumo gap mean?
more mobile pi electrons, therefore energy is more distributed over the molecule, stabilising it
ethene has 2 p electrons which can be in or out of phase
2 in or out of phase ethenes can combine in or out of phase, leading to 4 potential butadiene molecules
Why is butadiene more stable than ethene?
its homo lumo gap is smaller
What is pauling theory?
Pauling theory is the theory that the energy of a covalent bond between X and Y, is the mean of the energy of the covalent bond between X-X and Y-Y plus additional energy due to ionic factor (this additional energy is due to electronegativity)
Why is CCl3COOH a weak acid?
The Cl- are v electronegative. This means that they pull e- density towards themselves, reducing e- density on the COO-, stabilising it
What is a chiral compound?
a compound that isn’t superimposable on its mirror image
- chiral carbon has 4 different groups attached
How can you tell whether a compound is chiral?
How can you tell the difference between enantiomers?
What is a racemate
- chiral compounds rotate plane polarised light
- enantiomers rotate light by the same amount but in different directions
- racemates have no rotation
Equation for specific rotation? units?
SR = angle/conc*path length
conc in g/cm3
path length is dm
Dextro and laevo?
\+ = clockwise = dextro - = anticlockwise = laevm
Name S and R isomers
1) Assign priority on an atom by atom basis
2) triple>double>single
3) Face lowest priority away from you
4) draw arrow from 1st to 2nd to 3rd
5) clockwise = R, anticlockwise = S
What are diastereoisomers?
Stereoisomers that aren’t mirror images of each other and have more than one chiral centre
3 methods for separating racemates?
- separate crystals based on differences in their shapes
- use stereospecific nature of enzymes to remove one of the enantiomers, unreacted enantiomer can by removed by distillation
- Convert enantiomers into diastereoisomers and then resolve using ordinary separating techniques and then treat reagents to regenerate enantiomers
What are conformers?
different shapes of the same molecule caused by rotation around a single bond
Draw a staggered and eclipsed molecule? Which is lower energy?
eclipsed = higher energy staggered = higher energy
What is a rotational barrier? Draw a molecule with a high rotational energy and why do some molecules have a high rotational barrier?
- the activation energy required to rotate bonds
- High Ea to rotate bonds due to steric interference between groups
What is ring strain? What does a higher strain mean?
- The amount by which the bind angles arent 109.5
- more strain = higher energy
Between straight chain molecules there is a constant enthalpy of combustion, what does this show?
- molecules rotate to minimise strain, into their lowest energy configuration
- as cyclic molecules get bigger they can rotate to minimise strain
Describe SN1? draw graph
Describe the TS
What favours SN1?
- First order
- Rate is independent of the nucleophile
- 2 transition states and a carbocatian
- TS = 4 coordinate, 1 stretched bond
- Stereochemistry can be retained or inverted
- more stable carbocatian = increased rate of SN1
- polar solvents favour SN1 because TS/carbocatian are lower energy
- Tertiary Halogenoalkes more likely to SN1 because more stable carbocation
Describe SN2? Draw Graph
Describe the TS
What favours SN2?
- Second order
- Rate dependent on nucleophile
- TS = 5 coordinate, 2 stretched bonds
- Stereochemistry is inverted
- Large groups block Nu, decreasing SN2
- Primary halogenoalkanes favour SN2 (less steric hindrance, so Nu can get to molecule)
Why are strong bases bad leaving groups?
- don’t want to accept electrons
Why do aprotic solvents favour E2 and SN2
they are less likely to have hydrogens floating around to join carbocation in SN1/E1
Draw graph for E1 and E2
Fun
Draw SN1/SN2 mechanism
FUN
Draw E1/E2?
LEARN
Why are SN1 and E1 first ordeR?
second steps are fast and so arent the RDS
E1 VS SN1
- entropy favours E1 as there are more products
- stronger bases favour E1
- weak bases favour SN1
What happens when you replace C-H with C-D bond, with regards to mechanism rates?
rate of E2 goes down but E1 is unaffected. This is because the breaking of the C-D bond isnt in E1 RDS. Rate of E2 goes down becasue Ea for C-D bond is 5KJ/mol higher than C-H
What is regioselectivity?
Where the double bond ends up, forms a major and minor product
What is stereoselectivity?
arrangement of groups around double bond, a product based on kinetic/thermodynamic control
what is stereospecificity?
one isomer based on the mechanism and stereochemistry
E1 Vs E2
E1 is stereoselective and regioselective
E2 is stereospecific and regioselective
How do conformers affect carbocations? Draw pictures
configuration is eclipsed or staggered (steric hinderance)
elimination vs substitution
E1 and E2 are regioselective, therefore more substituted alkenes will go via elimination because the transition state will be more stable
What is antiperiplanar?
- configuration where hydrogen and LG are in the same plane but opposite sides
- This is the optimum configuration for E2
What is a stereoselective reaction?
give one a major product because the reaction has a pathway choice. Either a path with a lower activation energy (kinetic control) or one with a more stable product (thermodynamic control)
What is a stereospecific reaction?
Lead to the production of a single isomer due to the stereochemistry of the starting material and the mechanism of the reaction. NO CHOICE. The reaction gives a different diastereoisomer of the product from each stereoisomer of the starting material
What determines whether substitution or elimination?
- bulky bases favour elimination
- basicity (stronger favour elimination)
- temperature (entropy) favours elimination
- avoiding steric hinderance in the product (more stable product) release of strain favours elimination
- steric hinderance of delta + C favours elimination
What is regiospecificity
Forms one singular product
