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Organic Chemistry > Ch. 7 – Alkyl Halides & Nucleophilic Substitutions > Flashcards

Ch. 7 – Alkyl Halides & Nucleophilic Substitutions Flashcards

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1
Q

What are alkyl halides?

A

organic molecules containing a halogen atom (X) bonded to an sp3 hybridized carbon atom

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2
Q

What two types of halides involve sp2 hybridized carbons and are unreactive in Ch. 7 reactions?

A

vinyl and aryl halides

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3
Q

T/F: Alkyl halides undergo elimination reactions with Bronsted-Lowry bases.

A

True

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4
Q

T/F: A negatively charged nucleophile is always a weaker nucleophile than its conjugate acid.

A

False, it is always a STRONGER nucleophile than its conjugate acid.

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5
Q

T/F: Strong bases are needed for nucleophilic substitution.

A

False. A strong base would promote elimination.

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6
Q

T/F: Inductive effects always refer to the withdrawal of electron density?

A

False, alkyl halides can stabilize positive charge by induction.

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7
Q

What are vinyl halides?

A

alkyl halides that have a halogen atom bonded to C-C double bond

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8
Q

What are aryl halides?

A

alkyl halides that have a halogen bonded to a benzene ring

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9
Q

What are allylic halides?

A

have halogen bonded to carbon atom adjacent to C=C

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10
Q

What are benzylic halides?

A

have halogen bonded to the carbon adjacent to a benzene ring

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11
Q

T/F: allylic and benzylic halides are reactive in Ch. 7 reactions.

A

True! they are bonded to sp3 carbon!

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12
Q

How are alkyl halides named?

A

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

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13
Q

What is the polarity of alkyl halides?

A

weakly polar, has dipole dipole interactions because C-X bond is polar

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14
Q

Is there H-bonding in alkyl halides?

A

no

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15
Q

What is the trend for boiling point and melting point for alkyl halides?

A

BP and MP increase as size of R increases
BP and MP increase as size of X increases
(larger = more polarizable)

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16
Q

What is solubility for alkyl halides?

A

soluble in organic solvents, not in water

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17
Q

T/F: simple alkyl halides are good solvents

A

True because they are not flammable and they dissolve lots of organic compounds

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18
Q

What determines chemistry of alkyl halide?

A

polarity of C-X bond

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19
Q

What are the most common reactions for alkyl halides?

A

substitution and elimination

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20
Q

What do alkyl halides react with?

A

electron rich agents (nucleophiles)

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21
Q

What three things must be true for a nucleophilic substitution reaction to take place?

A

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

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22
Q

T/F: nucleophilic substitution reactions are Lewis acid-base reactions

A

True, nucleophiles act as base and donate electron pair to alkyl halide (lewis acid)

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23
Q

What is true about the product of a nucleophilic substitution rxn if the nucleophile starting material is neutral?

A

the product will have a positive charge

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24
Q

When does proton transfer occur of a NS reaction product?

A

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

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25
Q

What makes a good leaving group?

A

good leaving groups are weak bases
weaker base = better leaving group
(more stable leaving group = better able to accept electrons)

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26
Q

Is a base with a strong conjugate acid a good leaving gorup?

A

yes, weaker base

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27
Q

What does equilib favor in terms of leaving group?

A

equilib favors products more when leaving group is a weaker base than nucleophile

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28
Q

What do both nucleophiles and bases have?

A

lone pair and a pi bond

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29
Q

How do bases and nucleophiles differ?

A

differ in what they attack
base = attack protons
nucleophile = attack e- deficient atoms

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30
Q

T/F: strong base = strong nucleophile

A

true

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31
Q

T/F: a negatively charged nucleophile is only sometimes stronger than a conjugate acid

A

false, neg charge is always stronger than its conjugate acid

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32
Q

What is periodic trend for nucleophilicity?

A

nucleophilicity increases from right to left (F to H)

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33
Q

When does nucleophilicity not parallel basicity?

A

when steric hindrance is involved and when certain solvents are used

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34
Q

What is steric hindrance?

A

decrease in reactivity resulting from presence of bulky groups at site of atom… it is nucleophile to reach base (decreases nuc, but not basicity)

35
Q

What are nonnucleophilic bases?

A

sterically hindred bases that are poor nucleophiles

36
Q

What are solvent effects for nucleophiles?

A

nucleophilicity also depends on type of solvent

37
Q

What are polar protic solvents?

A

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

38
Q

How does polar protic solvent affect nucleophilicity?

A

nucleophilicity increases as size of nucleophile increases in protic solvents (smaller atoms are shielded by solvent)

39
Q

What are polar aprotic solvents?

A

solvents capable of dipole-dipole interactions but no H-bonding (no O-H or N-H)
can only dissolve cations (ion-dipole)

40
Q

What are naked anions?

A

anions not bonded to solvent (in aprotic solvent)

41
Q

How do aprotic solvents affect nucleophilicity?

A

in aprotic solvents, nucleophilicity parallels basicity

42
Q

What sigma bonds break and form during nuc sub rxn?

A

break: C-X
form: R-Nu

43
Q

in what order does bond making/breaking occur?

A

if one step, occurs at same time

if two steps, breaking occurs before making

44
Q

What is the Sn2 mechanism?

A

single step, second order, bimolecular reaction

45
Q

What is rate of Sn2 reaction?

A

rate = k[R-X][Nu]

46
Q

What is a concerted reaction?

A

bond breaking and bond making occur at same time (one energy barrier) Sn2

47
Q

What is the stereochemistry of Sn2 reaction?

A

backside attack, results in inversion of configuration

48
Q

What is frontside attack? What is final configuration?

A

nuc approaches from same side as leaving group (does not occur for Sn2) retains config

49
Q

What is backside attack? What is final config?

A

nuc approaches from opposite side of leaving group and results in inversion of configuration

50
Q

T/F: front/backside attack products are the same compound

A

false! they are enantiomers (diff compounds)

51
Q

Which actually occurs for Sn2 – backside or frontside attack?

A

backside attack, likely due to repelling of LG and nuc

52
Q

How is rate of Sn2 affected by number of R groups on C with leaving group?

A 
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
53
Q

What is the Sn1 reaction mechanism?

A

more than one step, first order kinetics, unimolecular, bond breaking occurs before bond making

54
Q

What is the rate of an Sn1 reaction?

A

rate = k[alkyl halide]

55
Q

What is the slow step in an Sn1 reaction?

A

first step, conversion to carbocation

56
Q

What is the first step of an Sn1 reaction?

A

heterolysis of C-X bond which forms a carbocation intermediate

57
Q

What is the second step of an Sn1 reaction?

A

nucleophile attacks carbocation and forms new C-Nu bond

58
Q

How many humps does the energy diagram for Sn1 have?

A

2, 2 energy barriers

59
Q

How many bonds does each transition state have for Sn1?

A

each has only one bond

60
Q

What is geometry and hybridization of carbocation with three groups?

A

sp2, trigonal planar (120º), contains vacant p orbital above/below plane

61
Q

What happens to stereochemistry for frontside attack?

A

dashes and wedges switch

62
Q

What happens to stereochemistry for backside attack?

A

wedges and dashes stay the same

63
Q

What does the loss of the leaving group generate in Sn1?

A

achiral carbocation that is planar

64
Q

Does front or backside attack occur for Sn1?

A

both equally, yields enantiomers to make racemic mixture

65
Q

What is racemization?

A

formation o equal amounts of two enantiomeric products from a single starting material

66
Q

When product contains to H’s and a + charge what happens?

A

loses additional proton

67
Q

How does the number of R groups on carbon affect rate of Sn1 reaction?

A

as number of R groups increase, so does rate of Sn1 reaction

68
Q

Which reactions work and don’t work for Sn1 (methyl to 3º)

A

methyl and 1º: don’t undergo Sn1
2º: slowly
3º: rapidly

69
Q

How does the number of R groups bonded to a carbocation affect its stability? Why?

A

as the number of R groups increases, stability of carbocation increases
Why? inductive effects

70
Q

What are inductive effects in terms of carbocation stability?

A

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
71
Q

What is hyperconjugation?

A

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)

72
Q

What is Hammond’s Postulate?

A

the transition state of a reaction resembles the structure of the species to which it is closer in energy (products or reactants)

73
Q

Is transition state of endothermic reaction closer to products or reactants?

A

products (products are higher in energy)

74
Q

Is transition state of exothermic reaction closer to products or reactants?

A

reactants (reactants are higher in energy)

75
Q

T/F: Anything that stabilizes a product in an endo reaction stabilizes transition state too and lowers is activation energy making the reaction faster

A

true! but in exo reaction, a more stable product may or may not form faster because Ea isn’t effected

76
Q

For endothermic reaction, does product in higher or lower energy form faster?

A

lower energy because it has lower Ea

77
Q

What are the four factors that determine if Sn1 or Sn2 mechanism occurs?

A

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

78
Q

What is the most important factor to determine Sn1/2 and what does it entail?

A

alkyl halide
increasing substitution = Sn1 (3º)
decreasing substitution = Sn2 (methyl + 1º)
** 2º can do either but will likely go Sn1

79
Q

How does nucleophile determine Sn1/2?

A

strong nucleophile = Sn2 (included in rate eq)
weak nucleophile = Sn1 (decreases rxn rate of competing Sn2 mech)
**Sn1 reaction rate is unaffected by nucleophile

80
Q

T/F: Most common nucleophiles in Sn2 have a negative charge

A

true

81
Q

How does the identity of the leaving group determine Sn1/2?

A

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)

82
Q

How does solvent identity determine if rxn will go Sn1/2?

A

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)

83
Q

Why are phosphorus compounds good leaving groups?

A

good leaving groups because they are weak, re-stabilized bases (go for both Sn1 and Sn2 reactions)

84
Q

Why don’t vinyl and aryl halides undergo Sn1 and Sn2 reactions?

A

they are sp2 hybridized

especially true for Sn1 because it would form HIGHLY unstable vinyl carbocation)

Organic Chemistry (26 decks)

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