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The addition of hydrogen bromide to unsymmetrical olefins, specifically pentene, and the resulting products. the theories of Kharasch, Lucas, and others to explain the predominance of 2-bromopentane over 3-bromopentane in the reaction. The document also mentions the use of different solvents and their effects on the reaction.
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THE ADDITION OF HYDROGEN BROMIDE
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A THESIS submitted to OREGON STATE COLLEGE
:tn the partial requirements fulfillment tor ' the ot
MASTER OF SCIENCE lune 195''
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TABLE OF CONTENTS ,
IN'lRODtJCTION , • • • , •
Page 1 PURPOSE OF INVESTIGATION • " , (^) ".... ... (^) • • • 10 EXPERIUENTAL • (^) • • • • •. ... (^) • • • • • • • • • • 13 Preparation ot 3•methylcyclohexene (^) • • • • • • 13 Preparation of 8uccin1mide • • • (^).... '. 13. Preparation ot N•bromosucc1n1mide (^) • • • • • 14
Preparation or 3-methyleyclohexene • • 16 Preparation of l-bromomethylcyclohexane , • (^) • • 17 The Infrared Analytical Method • • , • • • • • 18 the Addition of Hydrogen Bromide to 3•methylcyclohexene • • • • • • • • • .• 21 DISCUSSION OF RESULTS • • • • • , • • , • • • (^27) SUMMARY (^28) BIBLIOGRAPHY (^) • • • • • • • •. •-. (^) • • • • • • • • 29
LIST OF FIGURES AND TABlES
Page Figur 1- Infrar~d. Speqtra of the Isomeric .Btomo• methylcyelohexane.s ,, ... • , • • • • • .. 24 Figure 2 - Infrared Sprectrt1m of a 5'0-5'0 W.Xt..lre · of t~e 2~b~Gmometbyleyelohexene and 3-bromomethylcyclohexane • • • .. • .. • 25'
Product <:>f Hydrogen Bromide and.. .. 3-methylcyelohexene in Glacial
H+ ~- + C + ·C- • ~+ -0- + H butyric acid being an example. ~ ~! Therefore, it hydro·gen bromide (or other unsymmetrical reagent) is added to pentene, the reaction 1s as !ollowsa H !.!.t..±~ H H H H +- + H (^) t-;t_ H H H + (^) + H. H + -C- + Ct ;;:C+-~ + ...C- +H+ H Br ~ H+ -C- -l ~ -t ·C~C+ -tHH ~ 9 e +H +H +H (^) Br- • H ~H +H and 2-bromopentane would be the product. Lucas and Jameson (14 1 p.2476) disagreed ~ith the · theory of alternate polarizat .ion of carbon atoms, and. advanced their own theory ot electron displacement.
substituents upon the strength of organic acids, and to show that the effect extends throughout the entire carbon chain. Lucas a~d Jameson applied the work of Lewis to propylene, to acrylic acid and to dimethyl allene, and found that the addition of hydrogen halides could be explained by the theory of electronic displacement. The work of Lewis suggests that the alkyl group is more positive than hydrogen, while the carboxyl and halogen groups are more negative •. · Because or the positive char acter of the alkyl group as compared to hydrogent the structural formula or propylene can be wri tten as H H .. .. H'. Haf:Ca sCsH , wi th the elect~ constituting the H double bond being closer to the terminal carbon atom.
Addition of an unsymmetrical reagent would thus result in the more negative reagent attaching to the number two carbon. This theory also explains the rearrangement ot l·bromobutane to 2•bromobutane. Application of this theory to the addition of hydrogen bromide to pentene·2 would lead to a predominace of 3-bromopentane. The mixture ot 2• and 3 ... bromopentanes resulting from the addition reaction cannot be separated bY physical means. Lucas, Simpson and Carter (17, p.l468) found a slight difference in the refractive indices of the two isomers (2-br'omo, IlJ) 1.4416; the 3-bromo, no 1.4443 >, a.nd showed that the retractiye indices ot a mixture ot the pure compounds is a linear tunction of the composi tion or the mixture. Lucas and Moyse (15,pp.l460-1461) prepared the pentene-2 by the action of alcoholic potash on 3-bromo• pentane , and added hydrogen bromide to the olefin using glacial acetic acid as solvent. Using the refractive index method of analysis, they found ?8% of the 3-bromo isomer and 22% of the 2-bromo isomer. Lucas then con• sidered the theory of electronic displacement confirmed. Kharasch (lQ, p.408) determined electronegativity of some radi~als, and agreed with Lucas that the methyl group has a stronger pull on electrons than the ethyl group, but disagreed with his interpretation of the resul~ Kharasch says that the carbon atom in the ethylenic
lengths of time to transform the olefin to its electromer, he round the addition product varied from 63~ 3-bromo pentane and 37% 2-bromopentane to 16% 3-bromopentane and 84~ 2-bromopentane depending upon the length or time heated. The composition was determined by the refractive index method. Sherrill (18, pp. 3034-304l; 19, pp.3023·3033) and her associates prepared 2-pentene from 3-pentanol and. from 3-bromopentane. Realizing the possible existence of cis-trans isomerism in the pentene , they endeavored to separate the isomers using the technique of Van de alles (21 1 p.211) ot fractionating azeotrop1c mixtures ot pen tene with methanol. They obtained what seemed to be a reasonably pure isomer. The addition of hydrogen bromide to this isomer
pentane, and with glacial acetic acid as solvent gave 78% 3-bromopentane and 22% 2-bromopentane. Irradiation with ultraviolet light gave a product believed to · be an electromer because the addition product in absence ot solvent was 15% 3•bromo- and 85'% 2-bromopentane. 2-pentene was also made f'rom 2-pentanol and 2• bromopentane. The isomers were separated by fractionat1on of constant boiling mixtures with alcohol. This 2-pentene had properties slightly ditterent from those ot the olefin obtained trom 3~pentanol and 3-bromopentane. This isomer
added hydrogen bromide to give 93 - 95% 2 bromopentane, in no solvent. The use of glacial acetic acid as solvent. gave 85% of the 2-bromo compound. The 2-pentene derived from the 2·pentanol and 2-bromopentane appears to be the more stable electromer because irradiation ith ultra• violet light .altered properties of the olefin prepared from 3-bromopentane in the direction or those of the olefin derived trom the 2·bromopentane. Lauer and Stodola ( 12, p.l215) felt it necessary to find another method of analysis -because a small amount ot impurity in the addition product would cause large errors in results when used with the refractive index method. They devised a method based upon the conversion of the bromide to the anilide. The pure bromides were treated with magnesium and phenylisocyanate, and the resulting anilide used 1n the construction of a melting point curve. The addition product was converted to the. anilide and the melting point then indicated the composi tion... They used 3-bromopentane as the source of 2·pentene so that a check could be made with reference to the work of Sherrill and her associates. The addition of hydrogen bromide to this pentene and to one pr.epared from oc -ethyl crotonic acid was carried out in glacial acetic acid, in the absence or solvent, and in the pr·esence of perozide. The analysis indicated the same composition in each case, equivalent
'
general inductive effect of an alkyl group. The hypothesis assumes that the electron pair constituting a carbon hydrogen bond ot an alkyl group adjacent to an olefinic group or an electron deficient carbon atom is less local• ized than that · ot a carbon-carbon bond. A type of
H H H- ?- C rcu. (^) The number on carbon then would be H slightly more negative than the nuaber two carbon, and the addition of hydrogen bromide should re ult in a pre• dominance of the 2-bromo compound. Further proof from a physical-chemical basis has been listed b,y Deasy
methylacetylene, and the rate of bromination of certain alkyl substituted b nzenes. According to the theory of hyperconjugation, the addition of hydrogen bromide to pentene-2 would produce a predominanc or 2-bromopentane, since it is possible to write three hyperoonjugative structures involving the three « -hydrogens ot the methyl group, but only two forms involving the two ~ -hydrogens ot the ethyl group.
9 H I H H^ H I I^ I I (^) I I^ H-b·H H- C .. H (^) H- C - H H- C - H H - C - I H H- C - H H-^ C^ ...^ H^ H-^ -H H- C H - cse I^ H- csI^ e H- csI e I •^ •^ I^ 'C ..^ I 4 .. I H- C H^ -^ C^ H .. C^ H - C I II^ II^ H i) ~ H- y -H^ H-? ~ H-^ C • $^ H C-^ H H H H H
I
H H H I I I H-C-H H-C-H H-C-H
H - C - H H - C ~ H@ C - ·H
H ~ y H • rae H • Tse H-C-H I H-C·HI H•C-I H H H H
11 ot cyelohexene with the isopropyl and n-butyl groups, and recorded yields or 0 and 10 per cent respectively_ The infrared analytical method was used to deter~ mine the concentration of the isomers present, ~ogers (18) prepared 3-methyleyelohexene,. added hYdrogen bromide, and determined the infrared absorption curves ot the addition product. Rogers also prepared the pure 1-, 2-, and 3-bromomethyleyelohexanes and determined the infrared absorption curves on these isomers. It ~as found that a Beer • s law curve could be constructed trom data of mixtures of the 2- and ~·bromoeY:cloh~xanes, and this curve was used to determine the concentration or 2- and 3-bromomethyleyelohexane present. Because or the possibility of hydrogen bromide splitting from the ' 2-bromomethylcyclohexane and 1 then• adding in accordance with Markownikorr•s rule to the 1-methylcyclohexene formed to produce 1-bromomethyl eyclohexane, 1t wa$ decided to determine the amount ot tertiary bromide present in the sample. This was done by the metho~ of Wa lling, Kharasch, and Mayo (23 t p.2695')
the hydroly~ing agent. Determination of the amount of tertiary bromide present at regular intervals would give an indication of the rate of isomerization or the 2·bromo isomer to the l•bromo isomer.
12 Rogers yield on the addition of. hydrogen broJ,I'lide to 3-methylcyclohexene was 2; to 30 per cent. Since he was unable to draw any conclusions trom his results be· cause of low yield$, this work was initiated to improve the yields on the addition reaction in order to verity Rogers • work.
14 The crude s·uec1nim1de i s rec.r y stalliZed trom 95' per cent ethanol. Yieldt 210 - 230 grams (60 - ?2%) M.P. 124 - l25'^0 c. Preparation ot N•bromosuecinimid_e
directions or Ziegler ( 26., p.l09), but using smaller quantities ot reactants than given in the reference •.
placed in a solutt·on ct· 42 grams (1.0~ moles) or sodium hydroxide in 250 ml. water. As soon as the imide had
tla$k placed in an ice bath. l>ur1ng violent agitation of the solution, 53 ml. (2.08 moles) or liquid br-omine
!he preei.p1tate was rapidly separated trom the liquid br filtration, a·m. washed :oepe tedly with ice water to tree the precipitate of bromine. !he crude N•bromosuo<:1n1m1f!e· 1.s ~apidly recrystallizcu! from boiling water by pouring
stble using an aspirator and rubber dam. The product is dried and · stored in a '!facuum dess1eator over eoncentra.ted
M.P. 173.5 • l74.,0c., (reported 174.5- 175.5°C.) freparation of 3-brompcvglohexent N-bromosuccinimide is a brominating agent which brominates specifically in the alkyl position. !he re action or N-bromosuccinim1de with cyclohexene will give as product 3•bromooyclohexene (26, p.llO). A solution consisting or 5~5 ml. (5.0 moles) cyclohexene and 750 ml. carbon tetrachloride are placed in a 2-liter balloon flask equipped with an ettieient condenser. One hundred and seventy-eight grams (1.0 mole) N-bromosuocinimide is added, and heat applied to initiate the reaction. As soon as the reaction has begun, the beet is removed. Cooling will be needed to keep the reactants within the flask and condenser. After the reaction has subsided, heating is continued for an hour, and a tew· drops ot the solution is tested with an acidified potassium iodide solution. If a color is developed, heating is continued. After the reaction is complete, the solution 1s cooled and the succinimide is filtered from the solution. The carbon tetrachloride and cyclohexene are distilled from the solution, and the product vacuum distilled. Charcoal chips may be used as a boiling aid.
