Ch. 7 – Alkyl Halides & Nucleophilic Substitutions Flashcards
What are alkyl halides?
organic molecules containing a halogen atom (X) bonded to an sp3 hybridized carbon atom
What two types of halides involve sp2 hybridized carbons and are unreactive in Ch. 7 reactions?
vinyl and aryl halides
T/F: Alkyl halides undergo elimination reactions with Bronsted-Lowry bases.
True
T/F: A negatively charged nucleophile is always a weaker nucleophile than its conjugate acid.
False, it is always a STRONGER nucleophile than its conjugate acid.
T/F: Strong bases are needed for nucleophilic substitution.
False. A strong base would promote elimination.
T/F: Inductive effects always refer to the withdrawal of electron density?
False, alkyl halides can stabilize positive charge by induction.
What are vinyl halides?
alkyl halides that have a halogen atom bonded to C-C double bond
What are aryl halides?
alkyl halides that have a halogen bonded to a benzene ring
What are allylic halides?
have halogen bonded to carbon atom adjacent to C=C
What are benzylic halides?
have halogen bonded to the carbon adjacent to a benzene ring
T/F: allylic and benzylic halides are reactive in Ch. 7 reactions.
True! they are bonded to sp3 carbon!
How are alkyl halides named?
IUPAC: named as alkane with H-subs
change -ine to o
named alphabetically
Common: name all C atoms of molecule as single alkyl group then add space and alkyl group with -ide as ending
What is the polarity of alkyl halides?
weakly polar, has dipole dipole interactions because C-X bond is polar
Is there H-bonding in alkyl halides?
no
What is the trend for boiling point and melting point for alkyl halides?
BP and MP increase as size of R increases
BP and MP increase as size of X increases
(larger = more polarizable)
What is solubility for alkyl halides?
soluble in organic solvents, not in water
T/F: simple alkyl halides are good solvents
True because they are not flammable and they dissolve lots of organic compounds
What determines chemistry of alkyl halide?
polarity of C-X bond
What are the most common reactions for alkyl halides?
substitution and elimination
What do alkyl halides react with?
electron rich agents (nucleophiles)
What three things must be true for a nucleophilic substitution reaction to take place?
1) must have alkyl group with sp3 carbon bonded to X
2) must have a leaving group (X) that can accept electron density whe C-X bond breaks
3) must have nucleophile with lone pair or a pi bond
T/F: nucleophilic substitution reactions are Lewis acid-base reactions
True, nucleophiles act as base and donate electron pair to alkyl halide (lewis acid)
What is true about the product of a nucleophilic substitution rxn if the nucleophile starting material is neutral?
the product will have a positive charge
When does proton transfer occur of a NS reaction product?
when a NS product has a positive charge and a proton bonded to O or N, it readily loses a proton to form a neutral product
What makes a good leaving group?
good leaving groups are weak bases
weaker base = better leaving group
(more stable leaving group = better able to accept electrons)
Is a base with a strong conjugate acid a good leaving gorup?
yes, weaker base
What does equilib favor in terms of leaving group?
equilib favors products more when leaving group is a weaker base than nucleophile
What do both nucleophiles and bases have?
lone pair and a pi bond
How do bases and nucleophiles differ?
differ in what they attack
base = attack protons
nucleophile = attack e- deficient atoms
T/F: strong base = strong nucleophile
true
T/F: a negatively charged nucleophile is only sometimes stronger than a conjugate acid
false, neg charge is always stronger than its conjugate acid
What is periodic trend for nucleophilicity?
nucleophilicity increases from right to left (F to H)
When does nucleophilicity not parallel basicity?
when steric hindrance is involved and when certain solvents are used
What is steric hindrance?
decrease in reactivity resulting from presence of bulky groups at site of atom… it is nucleophile to reach base (decreases nuc, but not basicity)
What are nonnucleophilic bases?
sterically hindred bases that are poor nucleophiles
What are solvent effects for nucleophiles?
nucleophilicity also depends on type of solvent
What are polar protic solvents?
solvents that have O-H or N-H bonds that are capable of hydrogen bonding and can therefore solvate both cations and anions
cations = ion-dipole
anions = h-bonding
How does polar protic solvent affect nucleophilicity?
nucleophilicity increases as size of nucleophile increases in protic solvents (smaller atoms are shielded by solvent)
What are polar aprotic solvents?
solvents capable of dipole-dipole interactions but no H-bonding (no O-H or N-H)
can only dissolve cations (ion-dipole)
What are naked anions?
anions not bonded to solvent (in aprotic solvent)
How do aprotic solvents affect nucleophilicity?
in aprotic solvents, nucleophilicity parallels basicity
What sigma bonds break and form during nuc sub rxn?
break: C-X
form: R-Nu
in what order does bond making/breaking occur?
if one step, occurs at same time
if two steps, breaking occurs before making
What is the Sn2 mechanism?
single step, second order, bimolecular reaction
What is rate of Sn2 reaction?
rate = k[R-X][Nu]
What is a concerted reaction?
bond breaking and bond making occur at same time (one energy barrier) Sn2
What is the stereochemistry of Sn2 reaction?
backside attack, results in inversion of configuration
What is frontside attack? What is final configuration?
nuc approaches from same side as leaving group (does not occur for Sn2) retains config
What is backside attack? What is final config?
nuc approaches from opposite side of leaving group and results in inversion of configuration
T/F: front/backside attack products are the same compound
false! they are enantiomers (diff compounds)
Which actually occurs for Sn2 – backside or frontside attack?
backside attack, likely due to repelling of LG and nuc
How is rate of Sn2 affected by number of R groups on C with leaving group?
as number of R groups increases, rate of Sn2 reaction decreases methyl and 1º = fast 2º = go, slowly 3º = does not go why? steric effects
What is the Sn1 reaction mechanism?
more than one step, first order kinetics, unimolecular, bond breaking occurs before bond making
What is the rate of an Sn1 reaction?
rate = k[alkyl halide]
What is the slow step in an Sn1 reaction?
first step, conversion to carbocation
What is the first step of an Sn1 reaction?
heterolysis of C-X bond which forms a carbocation intermediate
What is the second step of an Sn1 reaction?
nucleophile attacks carbocation and forms new C-Nu bond
How many humps does the energy diagram for Sn1 have?
2, 2 energy barriers
How many bonds does each transition state have for Sn1?
each has only one bond
What is geometry and hybridization of carbocation with three groups?
sp2, trigonal planar (120º), contains vacant p orbital above/below plane
What happens to stereochemistry for frontside attack?
dashes and wedges switch
What happens to stereochemistry for backside attack?
wedges and dashes stay the same
What does the loss of the leaving group generate in Sn1?
achiral carbocation that is planar
Does front or backside attack occur for Sn1?
both equally, yields enantiomers to make racemic mixture
What is racemization?
formation o equal amounts of two enantiomeric products from a single starting material
When product contains to H’s and a + charge what happens?
loses additional proton
How does the number of R groups on carbon affect rate of Sn1 reaction?
as number of R groups increase, so does rate of Sn1 reaction
Which reactions work and don’t work for Sn1 (methyl to 3º)
methyl and 1º: don’t undergo Sn1
2º: slowly
3º: rapidly
How does the number of R groups bonded to a carbocation affect its stability? Why?
as the number of R groups increases, stability of carbocation increases
Why? inductive effects
What are inductive effects in terms of carbocation stability?
electronic effects that occur through sigma bonds
- electron donating R groups stabilize negative charge
- R groups are more polarizable than a hydrogen atom
- they can better donate electron density so positive charge is better dispensed
What is hyperconjugation?
spreading out charge by overlap of empty p orbital with adjacent sigma bond (C-C or C-H); delocalizes positive charge and stabilizes carbocation (carbocations with larger R groups are more stable)
What is Hammond’s Postulate?
the transition state of a reaction resembles the structure of the species to which it is closer in energy (products or reactants)
Is transition state of endothermic reaction closer to products or reactants?
products (products are higher in energy)
Is transition state of exothermic reaction closer to products or reactants?
reactants (reactants are higher in energy)
T/F: Anything that stabilizes a product in an endo reaction stabilizes transition state too and lowers is activation energy making the reaction faster
true! but in exo reaction, a more stable product may or may not form faster because Ea isn’t effected
For endothermic reaction, does product in higher or lower energy form faster?
lower energy because it has lower Ea
What are the four factors that determine if Sn1 or Sn2 mechanism occurs?
1) alkyl halide identity and number of R groups (most important)
2) nucleophile (strong or weak)
3) good or poor leaving group
4) polar protic or aprotic solvent
What is the most important factor to determine Sn1/2 and what does it entail?
alkyl halide
increasing substitution = Sn1 (3º)
decreasing substitution = Sn2 (methyl + 1º)
** 2º can do either but will likely go Sn1
How does nucleophile determine Sn1/2?
strong nucleophile = Sn2 (included in rate eq)
weak nucleophile = Sn1 (decreases rxn rate of competing Sn2 mech)
**Sn1 reaction rate is unaffected by nucleophile
T/F: Most common nucleophiles in Sn2 have a negative charge
true
How does the identity of the leaving group determine Sn1/2?
a better leaving group (weaker base) increases rate of BOTH Sn1 and Sn2 (breaking the C-X is important in both Sn1/2 rate equations)
How does solvent identity determine if rxn will go Sn1/2?
polar protic = Sn1 (solvates carbocation by ion-dipole and leaving group anion by H-bonding)
polar aprotic = Sn2 (makes nucleophile more nucleophilic because they are not hidden by binding to solvent and doesn’t hinder strong nucleophile)
Why are phosphorus compounds good leaving groups?
good leaving groups because they are weak, re-stabilized bases (go for both Sn1 and Sn2 reactions)
Why don’t vinyl and aryl halides undergo Sn1 and Sn2 reactions?
they are sp2 hybridized
especially true for Sn1 because it would form HIGHLY unstable vinyl carbocation)
