Orgo Prelim 3 – Flashcards
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Unlock answers| What are factors that Affect SN and E reactions? |
| Solvent, Nucleophilicity/Basicity, Leaving Group, and the Reactant |
| Which solvents are the best polar solvents? |
| SN and E reactions |
| What are some characteristics of SN2 reactions as polar solvents? |
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What are some characteristics of SN 1 reactions as polar solvents? |
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| What are some characteristics of E2 reactions as polar solvents? |
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What are some characteristics of E1 reactions as polar solvents? |
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| Is a charged atom (I-) higher or lower in nucleophilicity than an alcohol? |
higher.
because: charged species > lone pair e- > H2O or alcohol
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| What are some factors about polar protic solvents in terms of nucleophilicity? |
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What are some factors of polar aprotic solvents in terms of nucleophilicity? |
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| in terms of basicity, do weak bases have higher or lower pKa values? |
lower.
because CH3- > H2N- > RO - > F- > Cl- > Br- > I- pKa values decrease --> |
| How do SN2 reactions differ in polar protic and polar aprotic reactions in terms of nucleophilicity and basicity? |
In polar protic: nucleophilicity & basicity both decrease In polar aprotic: nuclephilicity increases and basicity decreases |
| For polar protic compounds (eg alkoxide anion), how do basicity and nucleophilicity differ in terms of carbon orders (teritiary, etc)? |
nucleophilicity: 1o > 2o > 3o basicity: 3o > 2o > 1o |
| What is the leaving group related to and which leaving group is the best? |
related to the stability of an atom; the best leaving group is a weak base=conjugate base of a strong acid
I- > Br- > Cl- > > F - |
| Which is better - a bigger leaving group or a smaller leaving group? Why? |
| The bigger leaving group is better because it has a delocalized charge. |
| In terms of an SN2 reactant, which carbon order is the strongest? What are two important factors? |
1o > 2o > > > (3o)
2 important factors: steric factor and branching Both slow the reaction to allow for a backside attack! |
| In terms of the reactant, what is the carbon order from highest to lowest? What must you also have? |
3o > 2o > 1o
must have beta-hydrogens and anti-configuration is preferred |
| What is the carbon order from highest to lowest in SN1 and E1 reactions? What factor is important? |
3o > 2o > > > (1o)
Electronic factor is important to stabilize carbozation |
| What you need to remember about SN2 reactions |
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| What you need to remember about E2 reactions |
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| What you need to know about SN1 reactions |
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| What to know about E1 reactions |
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What happens when a methyl halide or a primary alkyl halide reacts with a Lewis base? ; Na+CH3CH2O- + Br-CH2CH3--? |
The Lewis base replaces the halogen, which gets expelled as halide ion
---> CH3CH2O-CH2CH3 + Na+ Br- |
| What happens in a substitution reaction |
A Lewis base acting as a nucleophile donates an electron pair to an eletrophile to displace a leaving group.
Or in the words of Ganem: something goes in, something gets kicked out. |
| Show an example of substitution reaction (SN2 reaction) |
CH3O- + CH3-Cl -----> CH3-O-CH3 + Cl-
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| elimination reaction |
| reaction in which two or more groups are lost from within the same molecule |
| β-elimination |
| an elimination that involves the loss of two or more groups from adjacent carbons (conceptually the reverse of an addition to an alkene) |
| what are alpha-carbons and beta-carbons |
alpha-carbons: the carbon bearing the halogen (in alkyl halides)
beta-carbon: the carbons adjacent to the alpha-carbon |
| What promotes the beta-elimination reaction of an alkyl halide? What are usually used as solvents? |
| strong bases promote; the conjugate acid alcohols of these bases are usually used as solvents. |
| In the presence of strong Lewis bases, what are the typical reactions for primary and tertiary alkyl halides |
primary: substitution tertiary: β-elimination |
| what is the rate of a reaction a function of |
| the rate of a reaction is a function of the concentrations of the reactants (because molecules are more likely to collide at a high concetration than a low) |
| What is the rate law for A + B ---> C when both A and B double in concentration? What if only the concentration of A is double and B has no effect? |
both double: rate = k[A][B] only A doubles=k[A] |
| What is the overall kinetic order for a reaction |
the overall kinetic order for a reaction is the sum of the powers of all the concentrations in the rate law.
the kinetic order in each reactant is the power to which its concentration is raised in the rate law. |
| why is the rate law of a reaction so important |
| tells about the reaction mechanism, the concentration terms of the rate law indicate which atoms are present in the transtition state of the rate limiting step. |
| What does SN2 stand for |
S=substitution N=nucleophilic 2=bimolecular |
| what does bimolecular mean |
| the rate limiting step involves two species |
| what is the relationship between the rate law and the machanism |
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what three different ways can a substitution reaction occur at the stereocenter |
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| What would retention of the configuration at the asymmetric carbon result from? |
| if the nucleophile comes in to the same side as the leaving group leaves; known as frontside attack |
| what would cause inversion of configuration at the symmetric carbon |
| if the nucleophile comes in to one side and the leaving group leaves from the opposite side (3 other groups must invert); known as backside attack |
| which stereochemical configuration is generally observed in all SN2 reactions |
inversion of configuration is usually observed in SN2 reactions at carbon stereocenters.
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| why is backside substitution preferred in the SN2 reaction? |
| the orbital containing the donated the electron pair must initially interact with an inoccupied molecular orbital of the alkyl halide? |
| steric factor |
any effect on a chemical phenomenom, such as a reaction, caused by van der Waals repulsions
SN2 reactions of branched alkyl halides are retarded by a steric effect |
| why do elimination ractions compete with SN2 reactions of secondary and teriary alkyl halides |
secondary and teriary alkyl halides react so slowly in SN2 reactions that the rates of elimination raectionscompete with the rates of substitution.
the rates of the SN2 of teritary alkyl halides are so slow that elimination is the only reaction observed. |
| when are the less basic nucleophiles more nucleophilic |
| when the nucleophiles in the nucleophilic atom are from the same group but different periods of the periodic table |
| solvent proticity |
whether the solvent is protic, has the most effect on rate.
the change of solvent will have a great effect on the rate of its SN2 reactions depending on the strength of the ion |
| what strongly accelerates the SN2 reactions of nucleophiles |
| eliminating the possibility of hydrogen bonding |
| In E2 reactions, what do syn- and anti-elimination mean |
syn- the dihedral angle between the C-H and C-X bond is 0° (the H and V groups leave from the same side of the reference plane)
anti- the dihedral angle between the C-H and C-X bonds is 180° (H and X groups leave from opposite sides of the reference plane |
| What are three reasons anti-elimination is preferred |
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| what is usually the predominant product an an E2 reaction |
| the most stable alkene isomer (has the most alkyl substituents); these isomers are formed in the greatest amount |
| when does the greatest use of the E2 reaction for the preparation of alkenes occur |
| when the alkyl halide has only one type of beta-hydrogen and only 1 product is possible. |
| in a reaction with an alkyl halide with beta-hydrogens + a Lewis base, what reaction do you get if a Lewis base acts like a nuclephile? what if a Bronsted base? |
nucleophile: SN2 reaction Bronsted base: E2 reaction |
| what are the two variables that determine which reaction will be the major process |
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| when asked to predict how a given alkyl halide will react, what 3 major questions must you ask |
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| carbene |
| a species with a divalent carbon atom |
| alpha-elimination |
| elimination of 2 groups from the same atom |
