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Nomenclature and Properties of Alkyl Halides

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Presentation on theme: "Nomenclature and Properties of Alkyl Halides"— Presentation transcript:

1 Unit 4 – Alkyl Halides, Nucleophilic Substitution, and Elimination Reactions
Nomenclature and Properties of Alkyl Halides Synthesis of Alkyl Halides Reactions of Alkyl Halides Mechanisms of SN1, SN2, E1, and E2 Reactions Nucleophilicity, Substrate, and Leaving Group Effects

2 Alkyl Halides Alkyl halide:
a compound with a halogen atom bonded to one of the sp3 hybridized carbon atoms of an alkyl group Two types of names: IUPAC system Common names

3 Nomenclature 1-chloropropane bromocyclohexane IUPAC System:
Alkyl halides are named as an alkane with a halo-substituent: Review the rules for naming alkanes covered in Unit 2 1-chloropropane bromocyclohexane

4 Nomenclature n-propyl chloride Cyclohexyl bromide Common Names:
alkyl group name + halide n-propyl chloride Cyclohexyl bromide

5 Nomenclature CH2Cl2 CHCl3 Methylene chloride dichloromethane
Special common names: CH2X2 = methylene halide CHX3 = haloform CX4 = carbon tetrahalide CH2Cl2 CHCl3 Methylene chloride dichloromethane chloroform trichloromethane CCl4 Carbon tetrachloride tetrachloromethane

6 Types of Alkyl Halides Alkyl halides can be classified by the type of carbon atom the halogen is bonded to: primary halide (1o): halogen attached to a 1o carbon secondary halide (2o): halogen attached to a 2o carbon tertiary halide (3o): halogen attached to a 3o carbon

7 Types of Alkyl Halides Geminal dihalide:
2 halogens bonded to the same carbon atom Vicinal dihalide: 2 halogens bonded to adjacent carbon atoms

8 Other Organic Halides thyroxine benzylic carbon benzylic chloride
Aryl halide: halogen is attached directly to an aromatic ring Benzylic halide halogen is attached to a carbon that is attached to a benzene ring thyroxine benzylic carbon benzylic chloride

9 Other Organic Halides Allylic carbon Allylic chloride Allylic halide:
halogen is attached to a carbon that is attached to a C=C Allylic carbon Allylic chloride

10 Other Organic Halides Vinyl Halide:
halogen attached to a carbon that is part of a C=C Monomer for PVC Monomer for teflon

11 Uses of Alkyl Halides Anesthetics: Chloroform (CHCl3) toxic
carcinogenic (causes cancer) Solvents: CCl4 formerly used in dry cleaning CH2Cl2 formerly used to decaffeinate coffee liquid CO2 used now

12 Uses of Alkyl Halides DDT Chlordane (termites) Freons:
Freon-12: CF2Cl2 Freon-22: CHClF2 Freon-134a: Pesticides: DDT Chlordane (termites)

13 Physical Properties Boiling Point:
Compounds with higher MW’s and greater surface area (more linear) tend to have higher BP. BP increases as size of halogen increases F < Cl < Br < I BP decreases as branching increases

14 Physical Properties More dense than water Density:
Alkyl chlorides are common solvents for organic reactions. CH2Cl2 CHCl3 CCl4 More dense than water

15 Preparation of Alkyl Halides
Alkyl halides can be prepared from a variety of starting materials including alkanes, alkenes, alkynes, alcohols, and other alkyl halides. You are responsible for knowing and applying the synthesis of R-X by: free radical halogenation reactions free radical allylic bromination reactions

16 Preparation of Alkyl Halides
Free Radical Halogenation of Alkanes alkane + X alkyl halide(s) + HX Poor selectivity and moderate yields often limit usefulness. Bromination is more selective and gives the product formed from the most stable free radical. Chlorination is useful when only one type of reactive hydrogen is present hu or D

17 Preparation of Alkyl Halides
Useful Examples: hu 50 %

18 Preparation of Alkyl Halides
The following free radical halogenation is doomed to failure! The following addition reaction occurs instead:

19 Preparation of Alkyl Halides
Free Radical Allylic Bromination: where NBS = N-bromosuccinimide hn NBS

20 Preparation of Alkyl Halides
NBS is used to generate low levels of Br2 in situ. Minimizes addition of bromine across the C=C Allylic bromination is highly selective and occurs in the allylic position due to resonance stabilization of the resulting free radical.

21 Preparation of Alkyl Halides
Examples: hu hu

22 Reactions of RX Most reactions of alkyl halides involve breaking the C-X bond. Nucleophilic substitution Elimination The halogen serves as a leaving group in these reactions. the halogen leaves as X-, taking the bonding electrons with it d+ d-

23 Reactions of RX Nucleophilic substitution:
reaction in which a nucleophile replaces a leaving group Nucleophile: electron pair donor Leaving group: an atom or group of atoms that are lost during a substitution or elimination reaction retains both electrons from the original bond

24 Reactions of RX General Equation for Nucleophilic Substitution
The nucleophile can be neutral or negatively charged, but it must have at least one lone pair of electrons. Example:

25 Reactions of RX Elimination Reaction:
two substituents are lost from adjacent (usually) carbons, forming a new p bond Dehydrohalogenation: an elimination reaction in which H+ and X- are lost, forming an alkene CH3O-

26 Reactions of RX There are two common types of nucleophilic substitution reactions: SN1 reactions substitution, nucleophilic, unimolecular 3o, allylic, benzylic halides weak nucleophiles SN2 reactions substitution, nucleophilic, bimolecular methyl and 1o halides strong nucleophiles

27 Reactions of RX

28 Reactions of RX Common strong nucleophiles: hydroxide ion
alkoxide ions many amines iodide and bromide ions cyanide ion Common weak nucleophiles: water alcohols fluoride ion

29 SN2 Reactions The reaction between methyl iodide and hydroxide ion is a concerted reaction that takes places via an SN2 mechanism nucleophile Leaving group substrate product Substrate: the compound attacked by a reagent (nucleophile)

30 SN2 Reactions Concerted reaction:
a reaction that takes place in a single step with bonds breaking and forming simultaneously SN2: substitution, nucleophilic, bimolecular transition state of rate-determining step involves collision of 2 molecules 2nd order overall rate law Rate = k[RX][Nuc]

31 SN2 Reactions SN2 Mechanism:
Nucleophile attacks the back side of the electrophilic carbon, donating an e- pair to form a new bond Since carbon can only have 8 valence electrons, the C-X bond begins to break as the C-Nuc bond begins to form

32 SN2 Reactions SN2 Mechanism for the reaction of methyl iodide and hydroxide ion:

33 SN2 Reactions Reaction Energy Diagram:
large Ea due to 5-coordinate carbon atom in transition state no intermediates exothermic

34 SN2 Reactions SN2 reactions occur with inversion of configuration at the electrophilic carbon. The nucleophile attacks from the back side (the side opposite the leaving group). Back-side attack turns the tetrahedron of the carbon atom inside out.

35 SN2 Reactions Inversion of configuration:
a process in which the groups bonded to a chiral carbon are changed to the opposite spatial configuration: R S or S R

36 SN2 Reactions Example: Predict the product formed by the SN2 reaction between (S)-2-bromobutane and sodium cyanide. Draw the mechanism for the reaction.

37 SN2 Reactions The SN2 displacement reaction is a stereospecific reaction a reaction in which a specific stereoisomer reacts to give a specific diastereomer of the product

38 SN2 Reactions SN2 reactions occur under the following conditions
Nucleophile: strong, unhindered nucleophile OH- not H2O CH3O- not CH3OH CH3CH2O- not (CH3)3CO- Substrate: 1o or methyl alkyl halide (most favored) 2o alkyl halide (sometimes) 3o alkyl halides NEVER react via SN2

39 methyl > 1o > 2o >>>3o
SN2 Reactions The relative rate of reactivity of simple alkyl halides in SN2 reactions is: methyl > 1o > 2o >>>3o 3o alkyl halides do not react at all via an SN2 mechanism due to steric hinderance. The back side of the electrophilic carbon becomes increasingly hindered as the number or size of its substituents increases

40 SN2 Reactions Steric hinderance at the electrophilic carbon:

41 SN2 Reactions KNOW THESE! Be able to apply these!
SN2 reactions can be used to convert alkyl halides to other functional groups: RX + I - R-I RX + OH- R-OH RX + R’O- R-OR’ RX + NH3 R-NH3+ X- RX + xs NH3 R-NH2 RX + CN- R-CN RX + HS- R-SH RX + R’S- R-SR’ RX + R’COO- R’CO2R KNOW THESE! Be able to apply these!

42 SN2 Reactions Example: Predict the product of the following reactions:

43 SN2 Reactions Example: What reagent would you use to do the following reactions: ? ?

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