ch04 - alkanes - FIU

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Chapter 4Alkanes: Nomenclature,

Conformational Analysis, and an Introduction to Synthesis

Alkanes= saturated hydrocarbons

Simplest alkane = methane CH4

We can build additional alkanes by adding - CH2 – units

CnH 2n+2 = saturated = no double bonds, no rings

“Straight-chain” alkanes have a zig-zag orientation when they are in their most straight orientation

Shapes of AlkanesThere is no limit on length of chain

Polyethylene = an “infinite” alkane

At least one carbon attached to more than two others

Branched alkanes

Same molecular formula but different connectivity of atomsConstitutional isomers have different physical properties

(melting point, boiling point, densities etc.)

Many constitutional isomers are possible

5 “hexanes”

C6H14

2

The number of constitutional isomers possible for a given molecular formula rise rapidly with the number of carbons

Crude Petroleum Refining

straight- chain alkanes of different sizes

Cracking Isomerization Reforming

smaller alkanes

branched alkanes aromatics

A complex mixture of organic compounds, largely alkanes

US uses 17 M barrels/day, 75% for energy, 8% chemicals

Refineries not only separate the compounds, but “reform” the molecules by catalytic cracking and rearrangements

Source of alkanes = petroleum

Hyd rocarbon Fraction s from Petro leum

boi li ng rangeof fraction (oC)

s ize range name and use

below 20 C1 to C4n atural gas, bottl edgas, p etrochemicals

20 to 60 C5 to C6 p etroleu m "ether". s olvents

60 to 100 C6 to C7 l igroi n, sol ven ts

40 to 200 C5 to C10 s traigh t-ru n gasol in e

175 to 325 C12 to C18 k eros en e and jet fu el

250 to 400 C12 and h igh er gas oil , fu el oil and d iesel oi l

non volati leliq ui ds C20 an d h igh er min eral o il,

lu bri catin g oilnon volati le s oli ds

C20 and hig her paraffi n wax, asph al t, tar

A reaction of alkanes - Combustion

All hydrocarbons are combustible

Combine with oxygen and release energy

Heat of combustion is very high

2C8H18 + 25O2 16CO2(g) + 18H2O( l)

∆Hcomb = -5452 kJ/mol (or -47.8 kJ/g)

Some Octane Ratings ofHydrocarbons and Addit ives

Octane Ratingheptane 01-pentene 912,2,4-t rimethylpentane 100benzene 106methanol 107ethanol 108methyl t-butyl ether 116toluene 118

Octane boosters (anti -knock additives)

Smoother engine performance if combustion is slowed

For decades, used Tetraethyl lead, now banned

For past decade, MTBE was used, now also banned

Examples of “reforming” to raise octane

hexane

acid catalyst+

branched a lkanes

CH3CH2CH 2CH2CH2CH3CH3CH2CH2CHCH3

CH3

CH3CH2CHCH2CH3CH3

heptane

silica-aluminacatalyst, 500oC20 atm H2

+ 4H2CH3CH2CH2CH2CH2CH2CH3

CH3

isooctane = 100 (2,2,5-trimethylpentane) (vs heptane = 0)

3

t Early chemicals were given “common” or “trivial” names based on the source of the compound or a physical property

t The International Union of Pure and Applied Chemistry (IUPAC)started devising a systematic approach to nomenclature in 1892

t The fundamental principle in devising the system was that each different compound should have a unique unambiguous name

t The basis for all IUPAC nomenclature is the set of rules used for naming alkanes

IUPAC Nomenclature lNomenclature of Unbranched Alkanes

Nomenclature of Unbranched Alkanes – Learn the first ten!

The unbranched alkyl groups are obtained by removing one hydrogen from the alkane and named by replacing the -ane of the corresponding alkane with - yl

Nomenclature of Unbranched Alkyl groups

1. Locate the longest continuous chain of carbons; this is the parent chain and determines the parent name.

2. Number the longest chain beginning with the end of the chain nearer the substituent

3. Designate the location of the substituent

Nomenclature of Branched-Chain Alkanes (IUPAC)

7 carbons = a heptane

Nomenclature of Branched -Chain Alkanes (IUPAC) – cont.

4. When two or more substituents are present, give each substituent a number corresponding to its location on the longest chain

¶ Substituents are listed alphabetically

4-ethyl -3-methylheptane

4

Nomenclature of Branched -Chain Alkanes (IUPAC) – cont.

5. When two or more substituents are identical, use the prefixes di-, tri-, tetra- etc.

a) Commas are used to separate numbers from each otherb) Repeat the number if two substituents on same carbomc) The prefixes are used in alphabetical prioritization

4-ethyl -2,3,3-trimethylheptane

6. When two chains of equal length compete to be parent, choose the chain with the greatest number of substituents

7. When branching first occurs at an equal distance from either end of the parent chain, choose the name that gives the lower number at the first point of difference

Two alkyl groups can be derived from propane

Nomenclature of Branched Alkyl Chains

4-isopropylheptane

How many butyl groups are possible?

The neopentyl group is a common branched alkyl group

Hydrogenstake their classification from the carbon they are attached to

Classification of Hydrogen Atoms

5

What type of hydrogens in neopentane?

CH3-C-CH3

CH3

CH3

t In IUPAC nomenclature halides are named as substituents on the parent chainl Halo and alkyl substituents are considered to be of equal ranking

t In common nomenclature the simple haloalkanes are named as alkyl halidesl Common nomenclature of simple alkyl halides is accepted by IUPAC

and still used

Nomenclature of Alkyl Halides

Nomenclature of Alcohols

In the IUPAC naming system, there may be asmany as four components to the name:

Locant indicates the position of a substituent.Prefix names the substituent group.

Pare nt is the parent a lkane.

Suffix names a key function.

Examples

CH3 CH2CHCH3

OHCH3CHCH2CH2OH

CH3

2-butanol 3-methyl-1-butanol

locant parentsuffix

locant prefix locantsuffix

parent

IUPAC Nomenclature of Alcohols

1. Select the longest chain containing the hydroxyl and change the suffix ending of the parent alkane from -e to -ol

2. Number the parent to give the hydroxyl the lowest possible number

3. The other substituents take their locations accordingly

4-methyl-1-hexanol

l Examples

l Common Names of simple alcohols are still often used and are approved by IUPAC

t Alcohols with two hydroxyls are called diols in IUPAC nomenclature and glycols in common nomenclature

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Nomenclature of cycloalkanes

t Ring compounds are very commont Named as cyclo + base name for number of C in ring

cyclobutane cyclopentane cyclooctane

t When one substituent is present it is assumed to be at position one and is not numbered

t When two alkyl substituents are present the one with alphabetical priority is given position 1l Numbering continues to give the other substituent the lowest

numberl Hydroxyl has higher priority than alkyl = position 1

Nomenclature of Substituted Cycloalkanes

Cycloalkyl group

t If other parts of the molecule are dominent or if a long chain is attached to a ring with fewer carbons, the cycloalkaneis considered the substituent

l Bicyloalkanes contain 2 fused or bridged ringsl The alkane with the same number of total carbons is used

as the parent and the prefix bicyclo- is used

l The number of carbons in each bridge is included in the middle of the name in square brackets

Bicyclic Compounds – the Playground of Organic Chemists

Beginning at bridgehead position, number around largest ring first, then second largest ring

Bicyclic Compounds – the Playground of Organic Chemists

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t Alkenes are named by finding the longest chain containing the double bond and changing the name of the parent alkane from -ane to -ene

t The compound is numbered to give one of the alkene carbons the lowest number

t The double bond of a cylcoalkene must be in position 1 and 2

Nomenclature of Alkenes and Cycloalkenes

The hydroxyl is the group with higher priority and must be given the lowest possible number

Double bonds plus alcohol hydroxyl groups = alkenols

Note that positions of both the C=C and the OH must be indicated

The vinyl and allyl groups are common names that need to be readily recognized:

Vinyl and allyl groups

l If two identical groups occur on the same side of the double bond the compound is cis

l If they are on opposite sides the compound is trans

l Several alkenes have common names which are recognized by IUPAC

Alkene nomenclature – cis and trans

(1) The name of the pa rent alkane is modified by dropping the "ane" ending and adding "yne."

(3) The location of the triple bond is g iven by the lower positional number of the a lkynyl carbons.

CH3CH2CH2C CH5 4 3 2 1

1-pentyne

CH3C CCH3

2-butyne

(2) The parent chain is numbered to give the carbons of the alkyne lower numbers .

Alkynes = named similarly to alkenes

CH3C CCH2Cl4 3 2 1

e1-chloro-2-butyn

CH3CH2CHC CHCH3

5 4 3 2 1

e3-methyl-1-pentyn

.(5) In an alkynol, the alcohol has priority in numbering

.(4) Positions of substituents are determined by the usual rules

HC CCH2CH2OH4 3 2 1

3-butyn-1-ol

C CH pKa = 25R

1-alkynes are also called terminal alkynes

Terminal alkynes have an acidic H

8

.

The unbranched alkanes (CH4, CH3CH3, CH 3CH2CH3, CH3CH2CH2CH 3, etc.)form a regular s eri es where each member di ffers from the next in order by -CH2

-. Such a regular series is called a homo logous s eri es

pentane hexane heptane o ctanebp (oC) 36 69 98 126

.

The b oili ng poin ts of the u nbranche d alkane s incre ase more or le ss regu larl y w i th in creasing si ze re flec tin g the i ncreasin g van de r Waals attrac tion s

Physical Properties of Alkanes

pentane hexane heptane octanemp (oC) -130 -95 -91 -57

∆ = 35 ∆ = 4 ∆ = 34

Melting points reflect size, but also packing factors

Propane: -188oC Ethane: -183oC Methane: -182o C

Other physical properties of hydrocarbons

Less dense than water

Immiscible with water and other highly polar solvents

Solvent for non-polar organic compounds

t Ethane has relatively free rotation around the carbon-carbon bond

t The staggered conformation has C-H bonds on adjacent carbons as far apart from each other as possiblel The drawing to the right is called a Newman projection

t The eclipsed conformation has all C-H bonds on adjacent carbons directly on top of each other

Sigma Bonds and Bond Rotation

How to draw a Newman projection

Sight down the C-C bond (sigma bond is symmetric)

Groups on front carbon intersect in center

Groups on back carbon end at circle

The staggered conformation is more stable than eclipsed by 12 kJ mol -1

The potential energy diagram of the conformations of ethane

Rotation rate 1011/sec

Energy barrier called torsionalstrain

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HC

CC

H

H

H

H H

H

HThe co nformationalfeatures are the samefor the two C-C bonds.

eclipsed staggered

rotationH

HH HH

CH3

HH

CH3H

H

H

Bond Rotations in Propane

2 equivalent C-C bonds

Consider as ethane with 1 H replaced by a CH3

Barrier = 14 kJ/mol

Conclusion: eclipsing of methyl with H adds very little to energy barrier

Analysis of the Extreme Conformations

During o ne complete rotation around the internal bond in butane, the three staggered conformations include two called "gauche" and one called "a nti."

120o

rotation

120o

rotationC CHH

H

CH3

CH3 HC CH

HH

CH3CH3

HC C

H

HH

CH3

CH3

H

The Newman projection formulas as viewed from the left are:

120o

rotation

120o

rotation

anti gauche ga uche

CH3

HH

HH

CH3

CH3

H

H

HH

CH3

CH3

H

H

HH

CH3stericstrain

.

The stag gered anti is more stable than the two equivalent stagg ered gauche conforma tions. In the a nti conformation, the t wo CH3 groups are on opposite sides of the structure. In t he ga uche co nformations, the two groups are within van der W aals repulsive intera ction distance, and 3.8 kJ/mo l of steric strain energy is introduced

Rotation around C2-C3 of butane gives three energy minimaThe gauche conformation is less stable than the anti conformation by

3.8 kJ mol-1 because of repulsive van der Waals forces between the two methyls

12 0o

rotation

120o

ro tation

anti gauche gauche

CH3

HH

HH

CH3

CH3

H

H

HH

CH3

CH3

H

H

HH

CH3

Conformational Analysis of Butane

120o

rotation

120o

rotationC CHH

H

CH3

CH3 HC CH

HH

CH3CH3

HC C

H

HH

CH3

CH3

H

3 minima and energy barriers between each (3 maxima)

Conformational Analysis of Butane

t Heats of combustion per CH2 unit reveal cyclohexane has no ring strain and other cycloalkanes have some ring strain

Relative Stabilities of Cycloalkanes: Ring Strain

t Angle strain is caused by bond angles different from 109.5o

l C-C angle of 60 means orbital overlap is reducedt Torsional strain is caused by eclipsing C-H bonds on adjacent C’s

l Cyclopropane has both high angle and torsional strain

The Origin of Ring Strain in Cyclopropane :

Angle Strain and Torsional Strain

C

C C

HH

HHH H

Normal bond length sp 3-sp3C-C = 1.54 Å

Normal sp3C-H =1.10 Å sp2C-H=1.09 Å

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t Cyclobutane has considerable angle straint It bends to relieve some torsional strain

Cyclobutane: Angle Strain and Torsional Strain

.

The internal angles of a regular pentagon are 108o, close to the idealized tetrahedral bond angles . Thus, a planar cyclopentane would have very littl e angle strain

108o

But a planar geometry would have very severe torsional strain (10 eclipsed H). Consequently, the geometry of cyclopentane is bent.

bent or puckered geometry

H

HH

HH

H

H

HH

H

The torsional strain is reduced inthe bent structure. Four H are st illeclipsed, but 6 are staggered.

Cyclopentane

t Not 120o, the angle of simple hexagont Adopts a chair conformation with no ring strain!t All bond angles are 109.5o and all C-H bonds are perfectly

staggered

Cyclohexane: the perfect ring

1 2

3

45

6 HH

HH

H

HH

HH

H

HH

Newman projections alongthe C1-C6 and C3-C4 bondsshow the different orientationsof the axial and equatorial H. 5

2

1 3

CH2

CH2

H

H

H

H

H

H

H

Haxialeq

Two types of H in chair cyclohexane

6 H are axial (up and down)

6 H are equatorial(close to plane of ring)

t The boat conformation is less stable because of flagpole interactions and torsional strain along the bottom of the boat

Rotation about single bonds flips the ring to a boat form Ring flipping leads to a second chair

Proceeds via boat (or slightly twisted boat)

Every axial position becomes equatorial, and vice versa

Ring flipping occurs rapidly at room temperature

chair 1 boat chair 2

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0

42 kJ/mol

chair

half-chair

boat

energetically preferredconformation is the tw is t boat

half-chair

chair

Conformational Energy Diagram for cyclohexane Another view of energy barrier

Flips about 100 x per sec

99.9% of molecules in chair form

Note: kcal/mol x 4.2 = kJ/mol

Energy and Equilibrium

Note that 5 kJ/mol produces nearly a 90:10 preference

t The C-C bonds and equatorial C-H bonds are all drawn in sets of parallel lines

èThe axial hydrogens are drawn straight up and down

Drawing Cyclohexanes

To draw chair cyclohexanes, follow these steps:

1.) Draw the carbon chair. 2.) Add the axial hydrogens.

3.) Draw the C1 and C4 equatorial hydrogens.

4.) Draw the remaining equatorial hydrogens.

Cyclohexane structure can be elaborated

Adamantane

C10H16

DiadamantaneC14H18

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Diamond = “infinite” cyclohexane rings

Methyl cyclohexane is more stable with the methyl equatoriall An axial methyl has an unfavorable 1,3-diaxial interaction with

axial C-H bonds 2 carbons away

Substituted cyclohexanes

More stable by 7.6 kJ/mol

95% eqat R.T.

1

23

axial-methylcyclohexane

4

56

H

H

HH

HH

HH

CH3

H H

H 1

view a long the C1-C2 bond

26

3

gauche CH3

H

H

H

CH 2

CH2

view along the C3-C2 bond

1

24

3

gaucheCH3

H

H

H

CH2

CH 2

Each gauche interaction introduces 3.75 kJ/mol of steric strain

Destabilization of axial groups due to steric interactions

In axial methylcyclohexane there are two gauche interactions

1 2

3

equatorial-methylcyclohexane

5

6

4H

H

H H

H

H

H

H

H

H

HCH3


anti1

23

6

H

HCH2

CH3CH2

H

3anti


2

4

1H

H

H

CH3

CH2

CH2

Equatorial groups are free of torsional strain

Equivalent to a staggered butane

The larger the group, the greater the equatorial preference

t-butylcyclohexane is >99.9% equatorial

Disubstituted cycloalkanes

Cl

Cl

ClH H

Cl

ClCl

With two substituents on the ring in different positions, configurational isomers arise

ClH Cl

H

e.g.: 1,2-dichlorocyclobutane

cisisomer

trans isomer

Cis: groups on same side of ringTrans: groups on opposite side of ring

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12

3H H

CH3 CH3 H

HCH3

CH3

cis trans

1,3-dimethylcyclopentanes

For 3, 4, or 5 membered rings, can treat as if planar

Three options for pairs of cis-trans stereoisomers

Configurational Isomers in Disubstituted Cyclohexanes

Trans-1,4-dimethylcylohexane prefers trans-diequatorial conformation

Preferred comformations

Note methyls on opposite sides of ring

Cis-1,4-dimethylcyclohexane exists in an axial-equatorial conformation

Note that both are on the same side of ring

upper side in this projection

CH3

CH3

H

H Since the two bonds to theCH3 groups are "upper", meaning same side, they are "cis."

CH3

CH3

H

H

Test Two: Flatten the Ring

CH3 CH3

H H

Since the CH3 groups areon the same side, they are"cis."

upper bond

upper bond

low er bond

low er bond

SQUEEZE IT

Confirming whether cis or trans

Test one: identify each substituent as on the upper or lower side of ring

Is diequatorial always trans?

CH3

CH3HH

H

H

CH3CH3

Di-equatorial isomer

1,4

1,3

1,2

trans

cis

trans

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1 2

1,2-diaxial 1,2-diequatorial

1

2CH3

CH3

H

H CH3CH3

H

H

Each axial m ethyl groupgenerates 2 x 3.75 = 7.5 kJ/m olof strain energy, for a totalof 15 kJ/mol.

But the energy difference between the 1,2 -axial and the 1,2-diequa torial co nformat ions is only 3 ga uche intera ctions or 3 x 3.7 5 = 1 1.25 kJ/mol.

:

There is one gauche interaction from the interacting CH3 groups

12 CH3

CH3

H

H

CH2

CH2

gauche

Trans 1,2 dimethylcyclohexane: an energy analysis

The bicyclic decalin system exists in non-interconvertible cisand trans forms

Bicyclicand Polycyclic Alkanes

Steroid backbone

cholesterol

1. Hydrogenation of Alkenes and Alkynes

Synthesis of Alkanes and Cycloalkanes

2. Reduction of Alkyl Halides


3. Alkylation of Terminal Alkynes l Alkynes can be subsequently hydrogenated to alkanes


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l The synthetic scheme is formulated working backwardfrom the target molecule to a simple starting material

l Often several schemes are possible

Retrosynthetic Analysis-Planning Organic Synthesis

ch04 - alkanes - FIU

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