Halogen derivatives of alkanes Pharmacy Student
Classification 1. Monohalogen Derivatives : The halogen derivatives containing one halogen atom in a molecule General formula CnH2n+1X e.g. CH3 Cl methyl Chloride 2. Dihalogen Derivatives : The halogenderivatives containing two halogen atom in a molecule General formulaCnH2n X2 e.g. CH2Cl2 Methylene Chloride CH2 Br – CH2Br Ethylene Chloride
Classification 3. Trihalogen Derivatives : The halogen derivatives containing three halogen atom in a molecule e.g. CHCl3 Trichloro methane 4. Tetrahalogen Derivatives : The halogen derivatives containing two halogen atom in a molecule CF4 Carbon Tetrafluoride e.g. CCl4 Carbon Tetrachloride
1. Alkyl mono halides : General molecular formula Cn H2n+1 X • CH3 – Cl • C2H5 – Cl
What Is an Alkyl Halide • An organic compound containing at least one carbon-halogen bond (C-X) X (F, Cl, Br, I) replaces H It can contain many C-X bonds Properties and some uses Fire-resistant solvents Refrigerants Pharmaceuticals and precursors
Alkyl Halides R-X (X = F, Cl, Br, I) Classificationof alkyl halides according to the class of the carbon that the halogen is attached to. RCH2-X R2CH-X R3C-X 1o 2o 3o
Alkyl Halides • Alkyl halides are organic molecules containing a halogen atom bonded to an sp3 hybridized carbon atom. • Alkyl halides are classified as primary (1°), secondary(2°), or tertiary(3°), depending on the number of carbons bonded to the carbon with the halogen atom. • The halogen atom in halides is often denoted by the symbol “X”.
2-NamingAlkyl Halides Find longest chain, name it as parent chain (Contains double or triple bond if present) Number from end nearest any substituent (alkyl or halogen)
3-Isomerism in alkyl halides • 1-Position isomerism: • Compounds having the same molecular formula but differ in the position of the halogen atom • C4H9Br→CH3CH2CH2CH2Br 1-bromobutane • CH3CHCH2CH3 2-bromobutane Br
2- Chain isomerism • Depends on the type of the carbon chain; • Straight or Branched. • CH3CHCH2 Briso-butylbromide CH3 • CH3CH2CH2CH2Br 1-bromobutane
3- OpticalIsomerism • Present in alkyl halides of asymmetrical carbon atom • CH3 CH3 • H ClCl H • CH2CH3 CH2CH3
4-Methods Of Preparation • 1-From Alcohol: by the action of HX, SOCl2 (thionyl chloride) or PCl5 : • C2H5-OH +HClZnCl2 C2H5Cl + H2O • CH3(OH)CHCH3 + SOCl2 C5H5N CH3(Cl)CHCH3 + Isopropanol isopropyl chloride SO2 + HCl • CH3OH + PCl5 CH3Cl + POCl3 + HCl phosphorous oxychloride
2-From Alkene: CH3CH CH2 + HBr CH3CHBrCH3 isoppropyl bromide 3- Halogenation of Alkanes RH + X2 RX + HX • explosive for F2 • exothermic for Cl2 and Br2 • endothermic for I2
Chlorination of Methane • carried out at high temperature (400 °C) • CH4 + Cl2 CH3Cl + HCl • CH3Cl + Cl2 CH2Cl2 + HCl • CH2Cl2 + Cl2 CHCl3 + HCl • CHCl3 + Cl2 CCl4 + HCl
Physical Properties • Alkyl halides are weak polar molecules. They exhibit dipole-dipole interactions because of their polar C—X bond, but because the rest of the molecule contains only C—C and C—H bonds, they are incapable of intermolecular hydrogen bonding.
REACTIONS OF ALKYL HALIDES Alkyl halides (R-X) undergo two types of reactions : substitution reactions and elimination reactions. In a substitution reaction, the X group in R-X is replaced by a different group, e.g. R-XR-OH +XӨ In an elimination reaction, the elements of H-X are eliminated from R-X; the product is very often an alkene.
This is a nucleophilic substitution or nucleophilic displacement reaction on which OH displaces Br. The C-Br bond is polar, and the carbon (⊕) is susceptible to attack by an anion or any other nucleophile. ӨOH is the nucleophile (species which “loves nuclei” or has an affinity for positive charges). BrӨ is the leaving group ALKYL HALIDES – Substitution reactions
CH3-CH2—Br + ӨOH CH3-CH2—OH + BrӨ The general reaction is: R-X + NuӨ R-Nu + XӨ These are ionic reactions. There are two possible ionic mechanisms for nucleophilic substitution, SN1 and SN2. S – substitution; N – nucleophilic; 1 – unimolecular (the rate determining, r.d.s., step entails one molecule); 2 – bimolecular (the rate determining step entails two species). ALKYL HALIDES – Substitution reactions
ALKYL HALIDESThe unimolecular (SN1) reaction (a) In the first step, R-X dissociates, forming a carbocation, R⊕, and the leaving group XӨ. This is a slow, rate determining step (r.d.s.) and entails only one species, R-X. (b) R⊕+ NuӨ R-Nu In the second step the carbocation and the nucleophile combine. This occurs rapidly. The overall reactionis R-X + NuӨ R-Nu + XӨ The rate of the reaction = k[R-X]
Other Aspects of SN1 Reactions The most important feature of SN1 reactions is the carbocation intermediate. A. Alkyl halides which form stable carbocationswill undergo SN1 reactions. 3o alkyl halides form 3ocarbocations (stable) and will undergo SN1 reactions.
Alkyl halides which form stable carbocations will undergo SN1 reactions. 2o alkyl halides form 2ocarbocations (fairly stable) and it undergo SN1 reactions. 1ocarbocations are unstable, 1o alkyl halides will not undergo SN1 reactions. Substitution reactions of 1o alkyl halides proceed via the SN2 mechanism.
ALKYL HALIDES: The bimolecular (SN2) reaction • NuӨ + R-X ⇋ ӨNu---R---XӨ The nucleophile and the alkyl halide combine to form a pentacoordinate transition state. This is the slow rate determining step (r.d.s); it entails two species, R-X and NuӨ . The dotted lines indicate partially formed or partially broken covalent bonds. • ӨNu---R---XӨ Nu-R + XӨ The pentacoordinate transition state dissociates to form the product, Nu-R, and the halide ion (the leaving group). The rate of the reaction = k[R-X][NuӨ] The rate is dependent of the concentration of two species; higher concentrations increase the frequency of molecula collisions.
ALKYL HALIDES: The bimolecular (SN2) reaction • NuӨ + R-X ⇋ ӨNu---R---XӨ The nucleophile and the alkyl halide combine to form a pentacoordinate transition state. This is the slow rate determining step (r.d.s); it entails two species, R-X and NuӨ . The dotted lines indicate partially formed or partially broken covalent bonds. • ӨNu---R---XӨ Nu-R + XӨ The pentacoordinate transition state dissociates to form the product, Nu-R, and the halide ion (the leaving group). The rate of the reaction = k[R-X][NuӨ] The rate is dependent of the concentration of two species; higher concentrations increase the frequency of molecular collisions.
• Reactivity of Alkyl Halide: • Due to highly polar nature of Cδ+− Clδbond ethyl chloride is highly reactive. • Therefore alkyl halides are considered as synthetic tools in the hands of organic chemistry. • Due to low bond dissociation energy, alkyl halides are more reactive. • The order of reactivity of alkyl halides is as follows : • R - Cl < R – Br < R – I CH3 • CH3CH2CH2CH2−Cl <CH3CHCH2CH3 <CH3CCl • PrimaryClSecondary CH3tertiary
R-X + aqueous alkali :OH- ROH + :X-alcohol Chemical Reaction Hydrolysis : With aqueous KOH ethyl chloride gives CH3CH2CH2-Br + KOH CH3CH2CH2-OH + KBr CH3CH2CH2-Br + NH3 CH3CH2CH2-NH2 + HBr • R-X + alcoholic ammonia:NH3 R-NH2 + HX primary amine
R-X +alcohlic pot. Cyanide:CN- R-CN + :X-alkyl cyanide (1) nitrile • R-CN + H2O RCOOH + NH3 (2) • CH3Cl+KCN CH3CN +KCl • CH3CN + H2O CH3COOH +NH3 • CH3Cl + CH3COOAg CH3COOCH3 + AgCl R’-X + RCOOAg RCOOR’ + AgXesters
R-X + sodium alkoxide:OR´- R-O-R´ + :X-ether • C2H5Cl + C2H5ONa C2H5 O C2H5 + NaCl • R-X + sodium sulfide:SR´ R-SR´ + :X-thio-ether • CH3Cl + Na2S CH3-S-CH3 + NaCl • Formation of Grignard’s reagent • R-X + Mg/ether RMgXGrignard’s reagent • C2H5 + Mg/ether C2H5MgI
Reaction with Grignard’s reagent: • R-X + R’MgX R- R’ +MgX2 • CH3Cl + CH3MgCl CH3-CH3 + MgCl2 • Wurtz reaction : • In the presence of dry ether two moles of ethyl chloride reacts with sodium to give butane. • R-X + 2Na R-R + 2 NaXAlkane • 2 CH3Cl + 2 Na CH3-CH3 + 2NaCl • Reduction: • R-X + H2/Pt R-H + HX Alkane • CH3Cl + H2/Pt CH4 + HCl
Elimination : In the presence of alcoholic KOH • C2H5-Cl + alc KOH C2H4 + HCl
Dihalogen derivatives CnH2nX2
Di-Halogen Derivatives Geminal Dihalides Vicinal Dihalides ene Alkyl dihalide idene Alkyl dihalide | CH3 | | CH2 CH CH2 CH2 | | | | Cl Cl Cl Cl Terminal Non - Terminal Cl Ethylene dichloride H propylenedichloride | (1,2 - Dichloroethane) (1,2 - Dichloro propane) | | C Cl | CH3 H3C Cl | C | | | CH3 Cl (Isopropylidene dichloride) (Ethylidenedichloride) 2,2 - Dichloropropane 1,1 - Dichloroethane
Di-Halogen Derivatives ii) Preparation of Ethylene Dichloride a ) Addition of Chlorine to Ethylene CCl4 + CH2 CH2 Cl — Cl(g) CH2 = | CH2 | | Cl Cl Ethylene Ethylene dichloride 1,2-Dichloroethane
Di-Halogen Derivatives ii) Preparation of Ethylene Dichloride b ) Action of Ethylene Glycol and PCl5 + 2 HCl + 2POCl3 H2 C — CH2 H2 C — CH2 — — — — Cl OH OH Cl Ethylene dichloride + Cl Cl Cl Cl 1,2-Dichloroethane Cl— P Cl— P Cl Cl Cl Cl Ethylene glycol 1, 2-Ethanediol
Di-Halogen Derivatives iii) Preparation of Ethylidene Dichloride a ) Action of HCl and Acetylene addtn H—C C—H + H+ — Cl— H — C C — H = + H+ — Cl— — — H Cl excess Vinyl chloride Acetylene Ethyne Ethenylchloride H Cl — — H — C — C — H — — H Cl Ethylidene dichloride 1, 1-Dichloroethane
Di-Halogen Derivatives iii) Preparation of Ethylidene Dichloride b ) Action of Acetaldehyde with PCl5 H H Cl | Cl Cl | + C POCl3 CH3 | | Cl – P + C O | CH3 | Cl Cl | | Cl Phosphorus Pentachoride Acetaldehyde Ethanal Ethylidene Dichloride Phosphorus chloride
Di-Halogen Derivatives iv) Distinction between Vicinal & Geminal Dihalides by Hydrolysis reaction H2C CH2 | | | H2 C CH2 | boil Cl Cl + 2 KCl + | | Hydrolysis K K OH OH | | 1,2-Ethane diol (Glycol) (aq) OH OH Ethylene dichloride 1,2-Dichloroethane Hence aqalkali (NaOH /KOH) is used to distinguish between geminal and vicinal dihalides
Di-Halogen Derivatives H | | | Cl H3C C + K OH Boil | Hydrolysis K OH Cl – 2KCl Ethylidene dichloride 1,1-Dichloroethane H | H – H2O | = H3C C O | OH | | H3C C Acetaldehyde | OH Ethanal unstable It gives aldehyde or ketone depending on the position of the halogen atom
Tri-Halogen Derivatives A ] Chloroform ( CHCl3 ) ii) Oxidation of Ethyl alcohol | | H | Oxidation + 2HCl O H + Cl2 C C O | | | H3C H3C | | | H H Acetaldehyde Ethyl alcohol (Ethanal) Ethanol
Tri-Halogen Derivatives A ] Chloroform ( CHCl3 ) iii) Chlorination of Acetaldehyde | | Chlorination | | 3HCl CHO + CH3CHO 3Cl2 | + CCl3 Acetaldehyde Trichloroacetaldehyde Ethanol (Choral) iv) Hydrolysis of Choral H | H CCl3 – C O O Hydrolysis 2 CHCl3 + + (HCOO)2Ca Ca H Chloroform Calcium formate | (Trichloromethane) O H CCl3 – C O (Chloral) Calcium Hydroxide
Tri-Halogen Derivatives 1. Is Colorless , volatile , and Heavy liquid with sweet smell 2. Boiling point – 334 K 3. It is Insoluble in water but readily soluble in alcohol & ether 4. Is heavier than water 5. Produces unconsciousness when inhaled 6. Its vapour burns with a green edged flame 7. In liquid form , it is non-inflammable
Tri-Halogen Derivatives i) Oxidation Chloroform in presence of sunlight gives highly Poisonous gas phosgene carbonyl chloride hence: It is always stored In dark or amber colored bottles Sunlight + 2HCl O2 2COCl2 2CHCl3 + Air Phosgene Chloroform Carbonyl chloride Trichloromethane
Tri-Halogen Derivatives ii) Action with Concentrated nitric acid Cl Cl Cl C NO2 | NO2 | | HO H + C | | Cl - H2O Con. Cl Cl Nitro chloroform (chloropicrin) Chloroform CCl3 – NO2 is used as insecticide, tear gas.
Tri-Halogen Derivatives iv) Hydrolysis OH Cl Boil 3 K OH H C OH Cl + | H C | | | Unstable Hydrolysis (aq) OH Cl -3 KCl Chloroform – H2O Trichloromethane OH KOH O H | C = HCOOK H2O + Formic acid Potassium Formate Methanoic acid
Tri-Halogen Derivatives v) Hofmann’s Carbylamine Reaction NH2 NC warm 3H2O + + 3KCl 3 KOH CHCl3 + + (alc) (C6H5NH2) (C6H5NC) Phenyl isocyanide Aniline Phenyl Carbylamine Phenyl amine
Alcohols R-O-H Classification CH3, 1o, 2o, 3o Nomenclature: Common names: “alkyl alcohol” IUPAC: parent = longest continuous carbon chain containing the –OH group. alkane drop -e, add –ol prefix locant for –OH (lower number for OH)
Alcohols classified as: primary, 1o secondary, 2o tertiary, 3o according to their "degree of substitution." Degree of substitution is determined by counting the number of carbon atoms directly attached to the carbon that bears the hydroxyl group.
CH3 CH3CCH2CH2CH3 OH CH3CHCH2CH2CH2CH3 OH Substitutive Nomenclature of Alcohols Name as "alkanols." Replace -e ending of alkanename by -ol. Number chain in direction that gives lowest numberto the carbon that bears the —OH group. CH3CH2OH
CH3 CH3C CH2CH2CH3 OH CH3CHCH2CH2CH2CH3 OH Substitutive Nomenclature of Alcohols Name as "alkanols." Replace -e ending of alkanename by -ol. Number chain in direction that gives lowest numberto the carbon that bears the —OH group. CH3CH2OH Ethanol 2-Methyl-2-pentanol 2-Hexanol
CH3 CH3CHCH2CH2CH3 CH3CCH2CH2CH3 OH OH Classification H CH3CH2CH2CH2CH2OH OH primary alcohol secondary alcohol secondary alcohol tertiary alcohol