Org Ch 6
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Are halogens or carbons more electronegative?
What are polarized bonds prone to?
How does carbon-halogen bond lengths relate to bond strength?
As bond length increases, bond strength decreases
What is a Vinyl halide?
a halide bonded to a sp2, double bonded carbon
What is an Aryl halide?
a halide bonded to an sp2 hybridized carbon aromatic ring
What is an Acetylenic halide?
a halide bonded to sp hybridized, triple bonded carbon
What types of compounds are prone to under go nucleophilic substitutions and elimitation reactions?
Vinyl, Aryl, and Acetylenic halides
What is an SN reaction?
What are the steps of a nucleophilic substitution reaction?
- The Nu- donates an e- pair to the substrate
- The bond between C and LG breaks, giving borh e- from the bond to LG
- The Nu- uses its e- pair to form a new covalent bond with the substrate C
- The LG gains the pair of e- originally bonded in the substrate
What are the two types of SN reactions?
What is a nucleophile?
- A reagent that seeks a positive center
- Has an unshared pair of e-
What makes a good leaving group?
The substituent must be able to leave as a relatively stable, weakly basic molecule or ion
What is the rate of the substitution reaction based on?
The rate of the substitutio reaction is linearly dependent on the concentration of OH- and CH3Br
How can one measure the rate of nucleophilic substitution reaction?
- The consumption of the reactants
- The appearance of the products
How does the rate of reaction relate to the concentration of either reactant?
The rate of reaction is directly proportional to the concentration of either reactant
What is the mechanism for an SN2 reaction?
- Negative OH- brings an e- pair to partial positive C; partially negative Br begins to move away with an e- pair
- O-C bond partially formed; C-Br bond partially broken. Configuration of C begins to invert
- O-C bond formed; Br- departed. Configuration of C inverted
What is an exergonic reaction?
A reaction that proceeds with a negative free-energy change (releases energy to its surroundings)
What is an endergonic reaction?
A reaction that proceeds with a positive free-energy change (absorbs energy from its surroundings)
What is the Transition State Theory?
Most reactions should overcome an energy barrier known as activation energy (∆G) and form the transition state (T.S.)
What is the relationship between the equilibrium constant and ∆G°?
- ∆G° is the standard free energy change in a reaction
- this is the overall energy change of a reaction
- it is directly related to the equilibrium constant of a reaction
- if ∆G° is negative, products are facored at equilibrium (Keq>1)
- If ∆G° is positive, reactants are favored at equilibirum (Keq<1)
- If ∆G° is 0, products and reactants are equally favored (Keq=0)
What happens if ∆G° is negative?
Products are favored at equilibrium (Keq>1)
What happens if ∆G° is positive?
Reactants are favored at equilibrium; Keq<1
What happens if ∆G° is zero?
Products and reactants are equally favored; Keq=0
What is the reaction coordinate?
Indicated the progress of the reaction, in terms of the conversion fo reactants to products
What corresponds to the transition state?
The top of the energy curve
What is the free energy of activation (∆G+)?
The difference in energy between the reactants and the transition state
What is the free energy change for the reaction (∆G°)?
The difference in energy between the reactants and the products
How does a 10° temperature increase affect a reaction?
A 10° temperature will cause the reaction rate to double for many reactions taking place near room temperature
What is the relationship between (k) and ∆G+?
the relationship is exponential
What is k0?
Absolute rate constant, which equals the rate at which all transition states proceed to products (at 25°C k0=6.2*1012s-1)
What happens to the configuration of a molecule in an SN2 reaction?
Inversion of configuration
What does the rate of an SN1 reaction depend on?
- The rate of SN1 reactions depends only on concentration of the alkyl halide and is independent on concentration of the Nu-
What is the rate-determining step?
In a multistep reaction, the rate of the overall reaction is the same as the rate of the slowest step, known as the rate-determining step (r.d.s.)
What is Solvolysis?
An SN1 reaction where then ucleophile is a polar protic solvent
- Trigonal planar
- The central carbon atom in a carbocation is electron deficient; it is only six e- in its valence shell
- The p orbital of a carbocation contains no electrons, but it can accept an electron pair when the carbocation undergoes further reaction
What is the general order of reactivity towards SN1 reactions and carbocations?
How does the stability of the carbocation affect the rate of reaction?
The more stable the carbocation formed, the faster the SN1 reaction
What are the chances of an SN1 reaction happening from the right as opposed to the left?
There is a 50:50 chance that the attack will come from the right and the left
What factors affect the rate of SN1 and SN2 reactions?
- Structure of substrate
- Concentration and reactivity of the nucleophile (for SN2 reactions only)
- The effect of the solvent
- The nature of the leaving group
What is the general order of reactivity based on the structure of the substrate (towards SN2 reactions)?
Methyl>1°>2°>3°> vinyl or aryl
Which structures dont undergo SN2 reactions?
3°, vinyl and aryl
Which carbons do not undergo SN2 reactions?
sp2 and sp hybridized carbons
What is the general order of reactivity based on substrate structure in SN1 reactions?
Do allylic halides and benzylic hallides also undergo SN1 reactions?
Yes, at reasonable rates
How do you calculate the rate of SN2 reactions?
How does one measure the realtive strength of a Nu- (nucleophilicity)?
Measured in terms of the relative rate of its SN2 reaction with a given substrate
What is the relative strength of a Nu- correlated with?
- A negatively charged Nu- is always a more reactive Nu- than its conjugate acid
- In a group of Nu-s in which the nucleophilic atom is the same, nucleophilicities parallel basicities
- When the nucleophilic atoms are different, then nucleophilicities may not parallel basicities (type of solvent should be considered before comparing)
What are the 2 classifications of slovents?
- Polar solvents
- Non-polar solvents
What are the 2 types of polar solvents?
- Polar protic solvents
- Polar aprotic solvents
What are the best solvents for SN2 solvents?
Polar aprotic solvents, which have strong dipoles but do not have OH or NH groups
What do polar aprotic solvents do in SN2 reactions?
- Aprotic solvents tend to solvate metal cations rather than nucleophilic anions, and this results in 'naked' anions of the Nu- and makes the e- pair of the Nu more available
- Tremendous acceleration in SN2 reactions with polar aprotic solvents
What happens in SN2 reactions in Polar Protic Solvents?
In polar protic solvents, the Nu- anion is solvated by the surrounding protic solvent which makes the e- pair of the Nu- less available and thus less reactive in SN2 reactions
What makes a stronger Nu- in protic solvents?
I- is stronger than F- because its e- pair os more available to attack the substrate in the SN2 reaction
What do aprotic solvents solvate?
They do not solvate anions, but solvate cations
What do naked anions do?
Ace as the Nu-
Halid Nucleophilicity in Polar Aprotic Solvents
- Polar aprotic solvents do not solvate anions, but solvate cations
- The 'naked' anions act as the Nu-
- Since F- is smaller in size and the charge per surface area is larger than I-, the nucleophilicty of F- in this environment is greater than I-
How do solvents affect SN1 reactions?
- Solvent effects in SN1 reactions are due largely to stabilization or destabilization of the transition state
- Polar protic solvents stabilize the development of the polar transition state and thus accelerate this rate-determining step
What makes a good leaving group in SN1 and SN2 reactions?
The better a species can stabilize a negative charge, the better the LG in SN2 and SN1 reactions
Which compounds do not normally undergo SN2 reactions?
Which types of compounds are generally unreactive in SN1 or SN2 reactions?
Vinylic and Phenyl halides
What bases are used in Dehydrohalogenation?
Conjugate base of alcohols is often used as the base in dehydrohalogenations
What is the rate determining step i an E2 reaction?
Rate determining step involves both the alkyl halide and the alkoxide anion
What happens in an E2 reaction?
- EtO- removes a beta proton; C-H breaks; new pi bond forms and Br begins to depart
- Partial bonds in the transition state: C-H and C-Br bonds break, new pi C-C bond forms
- C=C is fully formed and the other products are EtOH and Br-
What is an E1 reaction?
What is the first step in an E1 reaction?
- Aided by the polar solvent, a chlorine departs with the e- pair that bonded it to the carbon
- Produces relatively stable 3° carbocation and a Cl-. The ions are solvated (and stabilized) by surrounding H2O molecules
What is the second step in an E1 reaction?
- H2O molecule removes one of the beta hydrogens which are acidic due to the adjacent positive charge. An e- pair moves in to form a double bond between the beta and alpha carbon atoms
- Produces alkene and hydronium ion
What does a primary substrate do?
With a strong base, favors SN2
What does a secondary substrate do?
With a strong base, favors E2
What does a tertiary substrate do?
With a strong base, favors E2 highly
What is the difference between small and bulky bases/Nu-?
small bases are unhindered, wheras bulky bases are hindered
Which reactions do weak bases favor?
- A weak base will follow a SN2 reaction to 100% completion
- A strong base will follow a SN2 reaction to 20% and an E2 reaction to 80%
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