1

SolutionstoProblemsAlkanes

BuildingBridgestoKnowledge

PhotoofawaterwaycanalinShanghai,China

1. NamethefollowingcompoundsusingtheIUPACsystemofnomenclature:

(a)

2,5-dimethylheptane

(b)

CH3 C

CH2CH3

CH2CH2 CH

H CH3

CH3

CH3 C CH2CH2 CHCH2

H CH3

CH3

CH

CH3 CH3

2

2,3,6-trimethyloctane

(c)

3-methylpentane(d)

3-methylhexane

(e)

4-ethyl-2-methylheptane

CH2CH3CH

CH3

CH2

CH3

CH3

CH3CH2CH

CH2CH2CH3

CH2CH3

CH3

CH3

CH2CH2CH3

CHCH2CH

3

(f)

2,4,4-trimethylhexane

(g)

3,4-dimethylhexane

(h)

3-ethyl-2-methylnonaneor3-isopropylnonane

(i)

5-ethyl-3,5-dimethylundecane

CH2CH3

CH3

CH3

CH3CHCH2CCH3

CH2CH3

CH3

CH3

CHCH

CH3CH2

CH3

CH3

CHCH

CH3(CH2)4CH2

CH3CH2

CH2CH3

CH3 CH3

CH3(CH2)4CH2

CH3CH2

C CH2CH

4

2. Writeabalancedchemicalequationforthefollowingreactions:(a)

(b)

majorproductminorproduct

(c)

(d)

n (2n + 2) 2C H + O →

n (2n + 2) 2 2 23n + 1C H + O n CO + (n + 1) H O

2⎛ ⎞ →⎜ ⎟⎝ ⎠

h3 2 3 2CH CH CH + Br ν⎯⎯→

CH3CH2C

Br

CH3

H + Mg(1) dry ether

H2O(2)

CH3C C CH2CH3 + H22Pt

5

(e)

(f)

(g)

(h)

CH3CH2C

Br

CH3 +

H

Na

CH3CH2CH3 + CH2N2hν

hνCH3

CH3

CH3

CH3C + C C OH

H

CH3CH2CH2Br + [(CH3)2CH]2CuLi

6

3. Suggestmechanismsforthefollowingconversions(Amechanismisaseriesofelementarystepsthatrationalizetheformationofproducts.Whenaddedtogether,thesumofthestepsfortheproposedmechanismshouldresultintheequationthatrepresentsthereactantsandtheproducts.)(a)

Theadditionofsteps2and3accountsforthereactantsandproducts.

(b)3CH3CH2Br+4Li+CuBr→CH3CH2CH2CH3+CH3CH2Cu+4LiBr

3. CH3CH2・+Li→CH3CH2Li3.CH3CH2・+Li→CH3CH2Li4.CH3CH2Li+CuBr→CH3CH2Cu+LiBr5.CH3CH2Cu+CH3CH2Li→(CH3CH2)2CuLi6.(CH3CH2)2CuLi+CH3CH2Br→CH3CH2CH2CH3+CH3CH2Cu+LiBr

Theadditionofsteps1-6resultsin

3CH3CH2Br+4Li+CuBr→CH3CH2CH2CH3+CH3CH2Cu+4LiBr(c)

h4 2 3CH + Cl CH Cl + HClν⎯⎯→

1. Cl2 hn⎯ →⎯ 2 Cli

2. Cl ⋅ + CH4 → CH3 ⋅

3. CH3 ⋅ + Cl2 → CH3Cl + Cl ⋅

1. CH3CH2Br + Li → [CH3CH2 Br]− . + Li+

1. CH3CH2Br + Li → [CH3CH2 Br]− . + Li+

2. [CH3CH2 Br]− . + Li+ → CH3CH2 ⋅ + LiBr2. [CH3CH2 Br]− . + Li+ → CH3CH2 ⋅ + LiBr

3 2 3 2 2 32 CH CH Br + 2 Na CH CH CH CH + 2NaBr→

1. CH3CH2Br + Na → [CH3CH2 Br]− . Na+

2. [CH3CH2 Br]− . Na+ → CH3CH2 ⋅ + NaBr

7

Theadditionofsteps1-5resultsin

4. Calculate∆Hfor

IfthebonddissociationenergiesinkJ/molare

bond bonddissociationenergy,kJ/molC-H 410Br-Br 192C-Br 280H-Br 364

3. CH3CH2Br + Na → [CH3CH2 Br]− - Na+

4. [CH3CH2 Br]− - Na+ → CH3CH2 ⋅ + NaBr

5. CH3CH2 ⋅ + CH3CH2 ⋅ → CH3CH2CH2CH3

3 2 3 2 2 32 CH CH Br + 2 Na CH CH CH CH + 2NaBr→

3 3 2 3 2CH CH + Br CH CH Br + HBr→

ΔHr = +410 kJ/mol + 192 kJ/mol -280 kJ/mol -364 kJ/molΔHr = -42 kJ/mol

8

5. Sketchapotentialenergyversusreactioncoordinatesdiagramforthefollowingreaction:

Mechanism:

Onthegraph,indicatethe:

(a) IntermediatesForthissystem,themorestableintermediateis

(b) enthalpyofreaction(c) energyofactivation,Eact(d) transitionstate

Thefirstactivatedcomplexatthefirsttransitionstateis

whereX=BrThesecondactivatedcomplexatthesecondtransitionstateis

CH3CH2CH3 + Br2hν

(CH3)2CHBr + HBr

1. Br2 hν⎯ →⎯ 2 Br ⋅2. CH3CH2CH3 + Br ⋅ → (CH3)CH ⋅ + HBr3. (CH3)3CH ⋅ + Br2 → (CH3)2 CHBr + Br ⋅

9

whereX=BrUsethefollowingdatatoapproximatethepotentialenergydiagram.

Bond bonddissociationenergy,kJ/mol

C-H 413Br-Br 192C-Br 280H-Br 364

10

5. Benzo[a]pyreneisacarcinogeniccompoundwhichhasbeendetectedingrilledsteaks.Combustionanalysisof3.22mgofbenzo[a]pyreneproduced11.25mgofCO2and1.38mgofH2O.(a)Calculatetheempiricalformulaofbenzo[a]pyrene.

(b)Inanexperiment,2.523gofbenzo[a]pyrenelowersthemeltingpointof50.0gramsofcamphorto170.6oC.Calculatethemolecularmassofthecarcinogen.

11.25mg x 12 g/mol44 g/mol

= 3.07 mg of C

1.38 mg x 2.0 g/mol18 g/mol

= 0.153 mg of H

3.07 mg12 mg/mmol

: 0.153 mg1.0 mg/mmol

= 1.67:1

C1.67H

C5H3

ΔTf = 40.0 K kgmol

m

451.6 K - 443.8 K = 40.0 K kgmol

x

2.523 gMW

0.050 kg7.8K

40.0 K kgmol

x 0.050 kg= 2.523 gMW

9.75 x 103 mol = 2.523 gMW

MW = 2.523 g9.75 x 103 mol

= 259 g/mol

11

(c) Calculatethemolecularformulaforbenzo[a]pyrene.

6. DrawtheNewmanprojectionformulasforCH3CHBrCH2CH3andsketchthe

potentialenergyversusrotationdiagramfora360orotationof2-bromobutanealongC2andC3carbonatoms.

(C5H3)x = 259 g/mol63 x = 259 g/molx = 4(C5H3)4 =C20H12

12

8. Suggestsynthesesforthefollowingcompoundsfromorganiccompoundscontainingnomorethanfourorfewercarbonatomsandanyothernecessaryinorganicreagents.

(a)(CH3)2CHCH3

(b)CH3CH2CH2CH2CH2CH2CH2CH3

(c)CH3CHDCH3whereDisdeuterium,anisotopeofH

(d) CH3CH2CH2CH2TwhereTistritium,anisotopeofH

13

9.Calculatethepercentageofmonochlorinatedproductsanticipatedinthemonochlorinationof2,4-dimethylpentane.GiveIUPACnamesofthemonosubstitutedchlorinatedcompounds.

ais1-chloro-2,4-dimethylpentanebis3-chloro-2,4-dimethylpentanecis2-chloro-2,4-dimethylpentane

StatisticalFactorxReactivityFactorPrimaryHydrogenAtomAbstraction12x1.0=12SecondaryHydrogenAtomAbstraction2x3.8=7.6TertiaryHydrogenAtomAbstraction2x5.0=10Percentof1-chloro-2,4-dimethylpentane

Percentof3-chloro-2,4-dimethylpentane

Percentof2-chloro-2,4-dimethylpentane

1229.6

x 100 = 40.5%

7.629.6

x 100 = 25.7%

14

10. Thefreezingpointof3.5gramsofanunknownorganiccompoundin100mL

ofbenzeneis5.0oC.Calculatethemolecularweightoftheunknownorganiccompound.

Thedensityofbenzeneis0.8765g/mL

1029.6

x 100 = 33.8%

ΔTf = Kf m

d = massvolume

0.8765 gmL

= mass100 mL

0.8765 gmL

x 100 mL = mass

87.65 g = mass

(278.7 − 278.2) K = 5.12 K kgmol

x

3.5 gMW

0.08765 kg

0.5 K

5.12 K kgmol

x 0.08765 kg = 3.5 gMW

0.00856 mol = 3.5 gMW

MW = 3.5 g0.00856 mol

= 410 g/mol