Module 3 thingz Flashcards
What is a covalent bond?
The bond formed when one electron on each atom is shared with the other
How do you find the formal charge?
Formal charge: Valence electrons - (bonds + dots)
How are sigma bonds made?
e.g. In h2 when two ls electrons end up in an orbital made from the overlap of the two 1s orbitals –> This is a sigma oribtal
What is a thiol functional group?
R-SH
What are constitutional isomers?
Isomers with a different order of attachment of their atoms
What are stereoisomers?
Isomers with the same order of attachment of atoms, but a different orientation of their atoms in space
What are enantiomers?
Steroisomers that are non superimposable mirror images of each other
What are diastereomers?
Stereoisomers that are not mirror images
What is the Z stereoisomer?
Bonds at a double bond are at the same side
What is the E stereosiomer?
Bonds at a double bond are on different sides
What are the E and Z stereoisomers rules?
- Priories assigned based on atomic number
- Where two atoms are the same, consider what is attached
- One of higher takes priorities over may of lower
- Double bonds treated at two single bonds
What is a chiral molecule?
Not superimposable on its mirror image is described as
being chiral. They contain 4 different groups attached.
–> Enantiomers
How do we distinguish between enantiomers?
Their interactions with plane polarised light.
–> They rotate plane polarised light in opposite directions
What is a racemic mixture?
A 1:1 mixture of two enantiomers
(+-)
–> Has no specific rotation, as they cancel out, so are not optically active
What are the rules for naming enantiomers?
- Identify the asymmetric C atom
- Assign priorites to the four molecules
- Draw the structure of the molecule viewed down the bin from C to to the lowest priority
- IF the arrangement of the remaining bonds from 1 to 3 appear clockwise assign R, if they appear anticlockwise assign S
What are diatereoisomers?
Occur in compounds with more than one asymmetric carbon (have a maximum of 2^n stereoisomers)
–> They do not have similar physical properties
What are organic molecules?
They are carbon based
What is homolytic bond cleavage?
One electron is transferred to each of the two atoms. The species generation are radicals (X. and Y.)
What is heterolytic bond cleavage?
Two electrons are transferred from the bond to one atom
Obtain charged intermediates
What is a Sn2 Nucleophilic sub reaction?
X leaves and Nu:- attaches
rate = k [C-X] [Nu:-]
–> Strong Nu can make up for a poor leaving group
–> Good Nu is essential
What are some features of the Sn2 transition state?
- Effective collision has taken place
- Corresponds to maximum free energy
- Nu and the X both loosely bound to C (partial bond)
- If its activation energy is high, reaction will be slow
What is a Sn1 reaction?
X leaves and then Nu:- attaches
rate = k[C-X]
–> Slow first step
–> Carbocation formed as the reaction intermediate, will react with any Nu
–> Major product from this reaction with most abundant Nu
–> Transition states with one extended bond
–> Strong leaving group is essential, strong Nu is not essential
What do Sn2 reactions produce and why?
One product which has inversion of stereochemistry, as Nu approaches from the back of the molecule, pushing the other atoms forward
What does the Sn1 reaction produce and why?
Two products with retention and inversion of stereochemistry
–> Nucleophile can approach and attach to planar intermediate (carbocation) from either side
What determined weather a reaction goes via Sn1 or Sn2?
Sn2
- -> methyl or primary (Easy if C groups are small)
- -> Fast if nucleophile is not sterically hindered
Sn1
- -> Rate of reaction increases with methyl < 1 < 2 < 3
- -> Alkyl groups are e donating (stabilise intermediate)
- -> Diminishes charge on C
Why is sn1 fast in polar solvents?
As the transition state can be stabilised by polar effect at C (positive) and H-bonding to leaving group (negative)
Why is Sn2 not fast in polar solvents?
The solvent will make the Nu more sterically hindered by bonging to it through H bonding and then it cannot attack in a Sn2 reaction
What are elimination reactions mechanisms determined by?
Base strength
strong base = E2
weak base = E1
What is a E1 reaction?
rate = k[C-Cl] Two step process --> Strong base is not required --> H-bonding solvents promote reaction --> E1 most common when leaving group is tertiary
What is a E2 reaction?
rate = k[CH3][OH-]
- -> Halide and base involved in single, rate determining step
- -> Must have a strong base e.g. OH
- -> Possible for primary, secondary or tertiary compounds
What are the differences between Sn2 and E2?
Sn2
- Requires backside attack
- Difficult if reaction centre crowded
- -> rate order 1 > 2 > 3
E2
- Easily accessible H+ is removed
- Little difference in ease of H+ removal for 1, 2 and 3
What are the steps of an addition reaction?
- HBr dissociates into H+ and Br-
- Slow addition of an H+ electrophile to the electron rich double bond (rate determining)
- Fast attack by a Nu at the carbocation intermediate
rate = k[H+][CH2=CH2]
What can a Br2 addition produce?
A bromonium carbocation intermediate
Are 1 prime or 2 prime carbocations more stable?
2 prime as they have more alkyl groups to stablise the intermediate
