《有机化学》课程教学课件（McMurry’s Organic Chemistry, 6th edition）Chapter 10 Alkyl Halides

Alkyl Halides Based on McMurry's Organic Chemistry,6th edition,Chapter 10

Alkyl Halides Based on McMurry’s Organic Chemistry, 6th edition, Chapter 10

What Is an Alkyl Halide ■ An organic compound containing at least one carbon- halogen bond(C-X) X(F,CI,Br,I)replaces H Properties and some uses CI F Br F C-H Fire-resistant solvents CI CI ■Refrigerants Trichloroethylene Halothane ■Pharmaceuticals and (a solvent) (an inhaled anesthetic) precursors F H Cl-C-F H-C-Br C H Dichlorodifluoromethane Bromomethane a refrigerant) (a fumigant)

What Is an Alkyl Halide „ An organic compound containing at least one carbon￾halogen bond (C-X) „ X (F, Cl, Br, I) replaces H „ Properties and some uses „ Fire-resistant solvents „ Refrigerants „ Pharmaceuticals and precursors

Naming Alkyl Halides Step 1:Find the longest chain and name it as the parent If a multiple bond is present,the parent must contain it. Step 2:Number the carbons of the parent chain beginning at the end nearer the first substituent,regardless of whether it is alkyl or halo. CH2 Br Br CH3 CH,CHCH2CHCHCH2CH3 CH CHCH2CHCHCH2CH3 23 4.56 123 4562 CHg CH3 5-Bromo-2.4-dimethvlheptane 2-Bromo-4.5-dimethvlheptane

Naming Alkyl Halides Step 1: Find the longest chain and name it as the parent. If a multiple bond is present, the parent must contain it. Step 2: Number the carbons of the parent chain beginning at the end nearer the first substituent, regardless of whether it is alkyl or halo

Naming with Multiple Halides ■If more than one of the CI CI same kind of halogen is present,use prefix CH CHCHCHCH2CH3 1 2345 6 di,tri,tetra CH3 ■If there are several different halogens, 2.3-Dichloro-4-methvlhexane number them and list them in alphabetical CH3 Br order H:CHCH2CH2CHCH3 543 2-Bromo-5-methylhexane (NOT 5-bromo-2-methylhexane)

Naming with Multiple Halides „ If more than one of the same kind of halogen is present, use prefix di, tri, tetra „ If there are several different halogens, number them and list them in alphabetical order

Structure of Alkyl Halides Halogens increase in size going down the periodic table, so the C-X bond is longer as you go down periodic table C-X bond is weaker as you go down periodic table C-X bond is polarized with slight positive on carbon and slight negative on halogen TABLE 10.1 A Comparison of the Halomethanes Bond length Bond strength Dipole moment Halomethane (pm) (k]/mol) (kcal/mol) (0) CHaF 139 452 108 1.85 CHCI 178 351 84 1.87 CHgBr 193 293 70 1.81 CHI 214 234 56 1.62

Structure of Alkyl Halides „ Halogens increase in size going down the periodic table, so the C-X bond is longer as you go down periodic table „ C-X bond is weaker as you go down periodic table „ C-X bond is polarized with slight positive on carbon and slight negative on halogen

Preparing Alkyl Halides Recall: Alkyl halide is made from the addition of HCI,HBr,HI to alkenes to give the Markovnikov product Alkyl dihalide are obtained from anti addition of bromine or chlorine to alkene HX H CH3 CH3 H CHg X=Clor Br X=Cl,Br,or I Thomson-Brooks Cole

Preparing Alkyl Halides Recall: „ Alkyl halide is made from the addition of HCl, HBr, HI to alkenes to give the Markovnikov product „ Alkyl dihalide are obtained from anti addition of bromine or chlorine to alkene

Reaction of Alkanes with Halogens Recall: Alkane Cl2 or Br2,heat or light replaces C-H with C- X but gives mixture of compounds Via free radical mechanism Hard to control CH4 Cl2 CH,C1+HCI Cla CH2Cl2 HCI L一CHCL3+HCI C→CCL4+HCI

Reaction of Alkanes with Halogens Recall: „ Alkane + Cl 2 or Br 2, heat or light replaces C-H with C￾X but gives mixture of compounds „ Via free radical mechanism „ Hard to control

Radical Halogenation of Alkanes If there is more than one type of hydrogen in an alkane,the reaction favors replacing the hydrogen at the most highly substituted carbons CI Dichloro-, CHCH2CH2CHa +Cl2 CH CH,CH2CH2CI CH CH2CHCH+ trichloro-, tetrachloro-. Butane 1-Chlorobutane 2-Chlorobutane and so on 30:70 2004 Thomson-Brooks/Cole H H R R一C一H H Primary Secondary Tertiary 1.0 3.5 5.0 Reactivity oo4nws-se

Radical Halogenation of Alkanes If there is more than one type of hydrogen in an alkane, the reaction favors replacing the hydrogen at the most highly substituted carbons

What are the reasons for the observed reactivity? A tertiary C-H bond is weaker than a secondary C-H bond, which in turn is weaker than a primary C-H bond less energy is needed to break a tertiary C-H bond than to break a primary or secondary C-H bond.The resultant tertiary radical is more stable than a primary or secondary radical 一RCHg -R2CH2 H H R RCH R-C. R R'一H+·CI H R R R'·+HCl Primary Secondary Tertiary -CI Stability Reaction progress

What are the reasons for the observed reactivity? A tertiary C-H bond is weaker than a secondary C-H bond, which in turn is weaker than a primary C-H bond @less energy is needed to break a tertiary C-H bond than to break a primary or secondary C-H bond. The resultant tertiary radical is more stable than a primary or secondary radical

Allylic Bromination of Alkenes 0 HH N-Br (NBS) ■N-bromosuccinimide Allylic (NBS)selectively positions brominates allylic positions HH Cyclohexene ■Requires light for 3-Bromocyclohexene (85%) activation H Br Br· HB Br2 Br Allylic radical HBr Br H

Allylic Bromination of Alkenes „ N-bromosuccinimide (NBS) selectively brominates allylic positions „ Requires light for activation