School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus Chemical Reactivity 120R. Organic Reactions

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1 Boiling point (o) School of hemistry and Physics, University of KwaZulu-atal, Westville ampus hemical eactivity 120 rganic ALKAES Saturated hydrocarbons - contain only carbon and hydrogen atoms, where each carbon forms four single(sigma) bonds with other atoms. Paraffins - old name for alkanes. Source - alkanes are isolated by fractional distillation from fossil fuels. rude oil/petroleum, a dark viscous liquid, is composed of a mixture of several hundred different hydrocarbons particularly straight chain alkanes, some branched alkanes, and varying quantities of aromatic hydrocarbons. The composition of the petroleum depends upon where it is found. The name petroleum means rock oil and is also called black gold. Table Product distribution in a typical distillation of crude petroleum & its uses %/V Bp / Atoms PDUTS USES 1-2 < atural gas Fuel, arbon black, polymerised petrol Petroleum ether(5, 6), Solvent, dry cleaning, refrigerant ligroin(7) Straight-run petrol* Motor fuel Kerosene, heater oil Furnace oils Gas oil, diesel fuel, lubricating oils, greases, Lubrication, diesel fuel, medicine ingredient waxes, asphalt 8-69 > 400 > 25 esidual oil, paraffin waxes, pitch, asphalt(tar) andles, waterproofing fabrics, roofing, protective paints, mfr rubber * refers to petrol straight from petroleum, without having being treated in any way. Boiling Points The longer chain alkanes are used as petrol, kerosene fuel, oils, greases, waxes, and asphalt(tar for roads). The longer chain alkanes obtained from crude petroleum are broken down into smaller fragments in a process called cracking. Figure Plot of Boiling points of alkanes versus chain length umber of atoms G. Dawson Page 1 of 10

2 1. ydrogenation of Alkenes/Alkynes using 2(g) over a Pt/Pd/i atalyst 2 Pt/Pd or i Addition occurs on the surface of the metal and is then always results in cis addition. 1. ombustion in the Presence of Excess 2(g) 2 5 (l) 3 2 (g) 2 2 (g) 3 2 (g) 2. Free adical alogenation of Alkanes to form Alkyl alides eat X 2 X or Light X where X2 = chlorine or bromine ALKEES & ALKYES Unsaturated ydrocarbons - & only, but containing double or triple bonds, as a result they could accommodate more hydrogen atoms. ompounds with more than one double or triple bond are called polyunsaturated. Many natural fats and oils are polyunsaturated note advertisments for margarines. lefins - old name 1. Dehydration of an Alcohol conc. 2 S o 2 B: Saytzeff s rule when deciding the major product 1. Electrophilic Addition of Alkenes a) Addition of alogens X 2 aloalkane X X (X = l or Br) G. Dawson Page 2 of 10

3 alogenation of an Alkyne to form an Alkyl alide Br Br 2 Br Br 3 Br Br Br Br E-isomer b) Addition of Polar eagents ydration of an Alkene to form an Alcohol 2 Alcohol ydrohalogenation of an Alkene to form an Alkyl alide X aloalkane X 2. atalytic eduction - ydrogenation a) Alkenes 2 Pt/Pd or i b) Alkynes - If 2 is added in the presence of Pt/Pd or i catalyst, the reaction will continue until the alkane is formed. If you wish to stop the hydrogenation at an alkene, a poisoned catalyst must be used, ie. Lindlars catalyst. 2 Lindlars cat. 3. zonolysis ( 3 ) 2 S Ketone where = any alkyl group G. Dawson Page 3 of 10

4 ALKYL ALIDES Although chlorine- and bromine - containing natural products have been isolated in marine species, most alkyl halides have been synthesised in laboratories. It is relatively easy to introduce halogens into various organic compounds. They are useful compounds with a variety of uses, namely, as synthetic reagents, solvents (l4, 2l2); insecticides and herbicides (DDT); fire retardants; cleaning fluids (l2=l2); refrigerants, and the coating used on non-stick cookware (Teflon ). 1. Free adical alogenation of Alkanes eat X 2 X or Light X where X2 = chlorine or bromine 1. ucleophilic Substitution Ether 2 Amine, 2 X a a Alcohol itrile 2. Dehydrohalogenation to form an Alkene Br K Strong Base 2 Br G. Dawson Page 4 of 10

5 ALLS & ETES Alcohols - compounds that have hydroxyl groups bonded to saturated sp 3 -hybridised carbon atoms. These compounds occur widely in nature and have many industrial and pharmaceutical applications. Two examples: ethanol is a fuel additive, an industrial solvent, and a beverage; and menthol, an alcohol isolated from peppermint oil, is a flavouring agent. ydrogen Bonding in Alcohols Alcohols are able to hydrogen bond which means that the intermolecular forces are stronger, the melting and boiling points of the alcohols are higher than the corresponding alkanes and ethers. In addition to higher melting and boiling points, the polar - group makes alcohols more soluble in water. The longer the nonpolar alkyl chain, the more insoluble it becomes. Acid-Base Properties of Alcohols Due to the lone pairs of electrons on the oxygen atom, an alcohol can act as a Lewis base: xonium ion arbocation The stability of the resulting carbocation is a major driving force in the reaction. 1. ydration of an Alkyl alide (ucleophilic Substitution) a or 2 X ax or X 2. Addition of Water to an Alkene 2 Alcohol 1. Formation of Alkoxide Ion a a 2 2. Dehydration of an Alcohol to form an Alkene conc. 2 S o 2 B: Saytzeff s rule when deciding the major product G. Dawson Page 5 of 10

6 3. xidation of an Alcohol to form an or Ketone a) Primary Alcohols Weak x. Agent P r 3 l P = pyridinium chlorochromate (weak oxidising agent) 2 Strong x. Agent r 3, Jones reagent arboxylic Acid ther strong oxidising agents include: K2r27, KMn4, 2r4. b) Secondary Alcohols 2 P or r 3, Ketone c) Tertiary Alcohols EATI! Ethers - compounds with two organic groups connected to a single oxygen atom. Ethers occur in nature and have industrial applications; for example, diethyl ether, the familiar "ether" of medicinal use, was once a popular anaesthetic agent but is now mainly used as an industrial solvent. 1. ucleophilc Substitution of an Alkyl alide 3 or X 3 ax or X a 3 ALDEYDES & KETES s and ketones occur widely in nature. Many have pleasant flavours and aromas, and are, therefore, used extensively in perfumes and consumer products, ie. Soaps, bleaches, air fresheners, etc. The functional group of aldehydes and ketones is the carbonyl group, in aldehydes this is always terminal, while in ketones it occurs within the parent chain. The reactions that aldehydes and ketones undergo are due to the nature of the carbonyl group. As a consequence of the bond polarity, most carbonyl reactions involve nucleophilic attack at the carbonyl carbon, accompanied by addition of a proton to the oxygen. G. Dawson Page 6 of 10

7 1. zonolysis of an Alkene to form an &/or a Ketone ( 3 ) 2 S Ketone 2. xidation of an Alcohol to form an or Ketone 2 P 2 P or r 3, Ketone 1. xidation of an to form a arboxylic Acid r 3, 2. eduction to form an Alcohol a) 1 o Alcohol LiAl 4 or ab o Alcohol b) Ketone 2 o Alcohol Ketone LiAl 4 or ab o Alcohol G. Dawson Page 7 of 10

8 ABXYLI AIDS arboxylic acids are the most important acyl derivatives in both nature and the laboratory. Vinegar, formed by the spoilage of wine, is a dilute solution of ethanoic acid; butanoic acid is responsible for the rancid odour of butter; and hexanoic acid is partially responsible for the unmistakable odour of goats. Acidic Properties pk a (acetic acid) = 4.74 at 25 o Acetic acid Acetate anion ydronium ion arboxylic acids are polar molecules which can hydrogen bond with themselves (to form a dimer) or with other molecules, they have high boiling points for their molar masses. Most carboxylic acids exist as dimers held together by hydrogen bonds: ydrogen bonds 1. xidation of an to form a arboxylic Acid r 3, 1. Acid-Base eaction to form a Salt a) ydroxide base, eg. sodium hydroxide 3 a 3 a 2 b) itrogen base, eg. ammonia or ethylamine, etc G. Dawson Page 8 of 10

9 2. Substitution a) Esterification ' ' 2 b) Acid hlorides Sl 2 l S 2 2 c) Acid Anhydrides 2 d) Amides ABXYLI AID DEIVATIVES The carboxylic acid derivatives are formed from the corresponding carboxylic acid by a nucleophilic substitution reaction. In comparing the acyl derivatives, the more polar a compound is, the more reactive it is. eactivity decreases from Acyl halides, acid anhydrides, esters to the least reactive, amides. f the derivatives, the most naturally occurring are the esters, which are pleasant smelling liquids responsible for the fragrant odours in fruits and plants. omenclature The IUPA system for naming carboxylic acid derivatives is summarised in the following table: DEIVATIVE Esters 2' Acyl alides X Acid Anhydrides 2' Amides 2 IUPA AME First give the name of the alkyl group attached to the oxygen, and then identify the carboxylic acid from which it is derived. Suffix: replace -ic acid with -oate Identify the acyl group and then the halide Suffix: replace -ic acid with -yl halide or -carboxylic acid with -carbonyl eplace the word acid with anhydride Unsubstituted (-2) replace the -oic acid or -ic acid with -amide, or by replacing the -carboxylic acid with carboxamide. G. Dawson Page 9 of 10

10 1. ucleophilic Substitution of arboxylic Acid See reactions section in arboxylic acids 2. Synthesis of an Amide from an Acyl hloride and an Amine 3 2 l l 1. ydrolysis of an Ester ' a 2 a ' AMIES Amines are organic derivatives of ammonia. Amines may be classified as either primary (2), secondary (2) or tertiary (3), depending upon the number of organic groups attached to the nitrogen atom. ote: this terminology is different from that used for the classification of alcohols. ompounds with four groups attached to the nitrogen atom are called quaternary ammonium salts, salts since the nitrogen must carry a positive charge. Ammonia 1 o 2 o 3 o Quaternary ammonium salt l Amines can be either alkyl-substituted (alkylamines) or aryl-substituted (arylamines), which exhibit some important differences in their chemistry. Because of the lone pair of electrons on the nitrogen, amines, like ammonia, can act as bases. 1. ucleophilic Substitution of an Alkyl alide 1. Synthesis of an Amide from both a arboxylic Acid or an Acyl hloride. G. Dawson Page 10 of 10