Oxygen Containing Reactions Flashcards
What does it mean when products have lower energy than the substrates
Change in energy is negative and the products are more thermodynamically stable than the reactants
What are electrophiles
Lewis acids are electrophiles
Accept electron pairs
3 features you can use to identify electrophiles
- Induction - a connected electronegative atom draws electron density away from the electrophilic centre
- Incomplete octet ***BF3
- Carbocation - + charge and electron deficient
Most common electrophiles
Aldehydes, ketones, carboxylic acid — all have carbonyl group
What are nucleophiles
Lewis bases
Donate electron pairs that come from lone pairs, multiple bonds and negative charges
*** look for the presence of lone pairs or electrons
Common nucleophiles
Water Alcohol Amine Amide Hydroxide Pi bond Halogen ions
Steps of an SN2 reaction
SINGLE CONCERTED STEP
Nuc attacks the electrophile at the same time the leaving group departs
Nuc comes from the back side and the configuration is inverted meaning its REALATIVE stereochemistry is inverted
Describe sn2 kinetics
Second order kinetics where the rate depends on the concentration of the substrate and the nucleophile
What does the rate of reaction depend on in SN2 reaction
Electrophile and nucleophile concentration therefore why its called SN2
How do large chemical groups affect SN2 reactions
Create steric hindrance and can block a nucleophile on its attack
This will greatly effect the speed of reaction ie a primary carbon will be quicker than a secondary
Tertiary often wont react under sn2 conditions
Trend of nucleophilicity on the period table
As electronegativity increases = nucleophile strength decreases therefore strength increases from left to right
Increases down a column
Factors affecting nucleophilic strength
Increases on periodic table from left to right and down
Negatively charged nucleophiles are more reactive than neutral
More sterically hindered will react slower
What is the different in nucleophilicity in halogens depending on the solvent
Aprotic solvent - strength increase going up ** usually this case
Polar protic solvent - strength increases going down (lots of H bonding)
What are some good leaving groups for SN2 reactions
Water I BR Cl Acetate Tosylate ion (Ts) Mesylate ion (Ms) Weak large bases
What are the favoured solvents for SN2 reactions and examples
Polar aprotic - no hydrogen bonding which normally reduces nucleophilicity
Acetone, ether, DMSO, DMF, THF, HMPA***
Describe the process in an SN1 reaction
Leaving group departs generating a carbocation
Nucleophile attacks p orbital
*** can be attacked from either side giving a racemic mixture
What is the kinetics of an SN1 reaction
First order where the rate depends on the concentration of the substrate but occurs over two steps (2 transition states)
The substrate is the most important factor in an SN1 reaction what are the best ones
Allylic and tertiary because they have the most stable carbocations
Secondary substrates can as well
How does the nucleophile and leaving group effect the reaction rate of an SN1 reaction
Nucleophiles do not affect rate because it occurs after the rate determining step
Good leaving group is necessary
What solvent is favoured in SN1 reaction
Polar protic because they stabilize the carbocation intermediate and hydrogen bond to the leaving group
*** anything that can form hydrogen bonds (OH) think sn1
What are the possible rearrangements of a carbocation in sn1
Methyl or H shift to more stable configuration
Will go from a secondary to a tertiary carbon `
