Estructura de__hidrocarburos

Estructura de hidrocarburos:

Alcanos

Clases de Hidrocarburos

HidrocarburosHidrocarburos

AromáticosAromáticosAlifáticosAlifáticos

HidrocarburosHidrocarburos

AromáticosAromáticosAlifáticosAlifáticos

AlcanosAlcanos AlquinosAlquinosAlquenosAlquenos

HidrocarburosHidrocarburos

AlifáticosAlifáticos

AlcanosAlcanos

Los alcanos son hidrocarburos en los cuales todos los enlaces son sencillos.

C CH H

H H

H H

HidrocarburosHidrocarburos

AlifáticosAlifáticos

AlquenosAlquenos

Los alquenos son hidrocarburos que contienen un doble enlace carbono-carbono.

C C

H H

H H

Hidrocarburos Hidrocarburos

AlifáticosAlifáticos

AlquinosAlquinos

Los alquinos son hidrocarburos que contienen un triple enlace carbono-carbono.

HC CH

HidrocarburosHidrocarburos

AromáticosAromáticos

Los hidrocarburos aromáticos más comúnes son los que contienen un anillo de benzeno.

H

H

H

H

H

H

CnH2n+2

Introducción a los Alcanos:Metano, Etano y Propano

Metano (CH4) CH4

Etano (C2H6) CH3CH3

Propano (C3H8) CH3CH2CH3

peb -160°C peb -89°C peb -42°C

Los Alcanos más Simples

Hibridación sp3 y

Enlaces en el Metano

Tetrahédrica

ángulos de enlace = 109.5°

longitud de enlace = 110 pm

sin embargo la estructura parece

inconsistente

con la configuración electrónica del carbono

Estructura del Metano

Configuración Electrónica del carbono

2s

2psolo dos electrones

desapareados

debe formar enlaces con solo dos átomos de

hidrógeno

los enlaces deben estar en

ángulo recto uno con

respecto

al otro

2s

2p

Se promueve un electrón del orbital 2s

al 2p

Hibridación Orbital sp3

30´s Linus Pauling

2s

2p 2p

2s

Hibridación Orbital sp3

2p

2s

Hibridación Orbital sp3

Mezclar (hibridizar) el orbital 2s y los tres orbitales 2p

2p

2s

Hibridación Orbital sp3

2 sp3

4 orbitales semillenos equivalentes son consistentes con cuatro enlaces y la geometría tetrahédrica

Hibridación Orbital sp3

Propiedades Nodales de los Orbitales

s

p + –

+

Forma de los orbitales híbridos sp3

s

p + –

+

Toma el orbital s y colócalo en la parte superior del orbital p

s + p + –+

Complemento de onda electrónica en regiones donde el signo es el mismo

Interferencia destructiva en regiones de signo opuesto

Forma de los orbitales híbridos sp3

híbrido sp

el orbital mostrado es híbrido sp

proceso analogo usando tres orbitales p y uno s da híbridos sp3

la forma de los híbridos sp3 es similar

+ –

Forma de los orbitales híbridos sp3

híbrido sp

- el orbital híbrido no es simétrico

- mayor probabilidad de encontrar un electrón en un lado del núcleo que en otro

- produce enlaces más fuertes

+ –

Forma de los orbitales híbridos sp3

–

+ –

El enlace C—H en el Metano

sp3s CH

H—C CH

produce un enlace .

Traslape en fase de un orbital semilleno 1s de hidrógeno con un orbital híbrido semilleno sp3 de carbono:

+

+

Justificación para la Hibridación Orbital

consistente con la estructura del metano

permite la formación de 4 enlaces en lugar de 2

los enlaces involucrados en los orbitales híbridos sp3

son

más fuertes que los involucrados en el traslape s-s o p-

p

Enlaces en el Etano

Estructura del Etano

CH3CH3

C2H6

geometría tetrahédrica en cada carbono

distancia de enlace C—H = 110 pm

distancia de enlace C—C = 153 pm

Traslape en fase de un orbital híbrido semilleno sp3 de un carbono con un orbital híbrido semilleno sp3

de otro.

El traslape es a lo largo del eje internuclear para dar un enlace .

El enlace C—C en el Etano

El enlace C—C en el Etano

Traslape en fase de un orbital híbrido semilleno sp3 de un carbono con un orbital híbrido semilleno sp3

de otro.

El traslape es a lo largo del eje internuclear para dar un enlace .

C4H10

Alcanos Isoméricos :Los Butanos

n-Butano CH3CH2CH2CH3

Isobutano (CH3)3CH

bp -0.4°C bp -10.2°C

n-Alcanos Superiores

CH3CH2CH2CH2CH2CH3

n-Pentano

n-Hexano

CH3CH2CH2CH2CH3

CH3CH2CH2CH2CH2CH2CH3

n-Heptano

Los Isómeros C5H12

n-Pentano

CH3CH2CH2CH2CH3

Isopentano

(CH3)2CHCH2CH3

Neopentano

(CH3)4C

C5H12

¿Cuántos isómeros?

El número de isómeros se incrementa al incrementar el número de carbonos.

No hay una manera sencilla de predecir cuántos isómeros hay para una fórmula molecular en particular.

Tabla 1 Número de Isómeros Constitucionales de

Alcanos

CH4 1

C2H6 1

C3H8 1

C4H10 2

C5H12 3

C6H14 5

C7H16 9

Tabla 1 Número de Isómeros Constitucionales de

Alcanos

CH4 1 C8H18 18

C2H6 1 C9H20 35

C3H8 1 C10H22 75

C4H10 2 C15H32 4,347

C5H12 3 C20H42 366,319

C6H14 5 C40H82 62,491,178,805,831

C7H16 9

Propiedades Físcas delos Alcanos y Cicloalcanos

Boiling Points of Alkanes

governed by strength of intermolecular attractive forces

alkanes are nonpolar, so dipole-dipole and dipole-induced dipole forces are absent

only forces of intermolecular attraction are induced dipole-induced dipole forces

Induced dipole-Induced dipole attractive forces

+–+

–

two nonpolar molecules

center of positive charge and center of negative charge coincide in each

+–+

–

movement of electrons creates an instantaneous dipole in one molecule (left)

Induced dipole-Induced dipole attractive forces

+–+–

temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)

Induced dipole-Induced dipole attractive forces

+–+ –

temporary dipole in one molecule (left) induces a complementary dipole in other molecule (right)

Induced dipole-Induced dipole attractive forces

+–+ –

the result is a small attractive force between the two molecules

Induced dipole-Induced dipole attractive forces

+– + –

the result is a small attractive force between the two molecules

Induced dipole-Induced dipole attractive forces

increase with increasing number of carbons

more atoms, more electrons, more opportunities for induced dipole-induceddipole forces

decrease with chain branching

branched molecules are more compact with

smaller surface area—fewer points of contact

with other molecules

Boiling Points

increase with increasing number of carbons

more atoms, more electrons, more opportunities for induced dipole-induceddipole forces

Heptanebp 98°C

Octanebp 125°C

Nonanebp 150°C

Boiling Points

decrease with chain branching

branched molecules are more compact with

smaller surface area—fewer points of contact

with other molecules

Octane: bp 125°C 2-Methylheptane: bp 118°C

2,2,3,3-Tetramethylbutane: bp 107°C

Boiling Points

All alkanes burn in air to givecarbon dioxide and water.

Propiedades Químicas:Combustión de Alcanos

increase with increasing number of carbons

more moles of O2 consumed, more

molesof CO2 and H2O formed

Heats of Combustion

4817 kJ/mol

5471 kJ/mol

6125 kJ/mol

654 kJ/mol

654 kJ/mol

Heptane

Octane

Nonane

Heats of Combustion

increase with increasing number of carbons

more moles of O2 consumed, more

molesof CO2 and H2O formed

decrease with chain branching

branched molecules are more stable(have less potential energy) than theirunbranched isomers

Heats of Combustion

5471 kJ/mol

5466 kJ/mol

5458 kJ/mol

5452 kJ/mol

5 kJ/mol

8 kJ/mol

6 kJ/mol

Heats of Combustion

Estructura de Alquenos

Alkenes

Alkenes are hydrocarbons that contain a carbon-carbon double bond

also called "olefins"

characterized by molecular formula CnH2n

said to be "unsaturated"

Hibridación sp2 y Enlaces en el Etileno

C2H4

H2C=CH2

planar

bond angles: close to 120°

bond distances: C—H = 110 pm

C=C = 134 pm

Structure of Ethylene

2s

2p

Promote an electron from the 2s

to the 2p orbital

sp2 Orbital Hybridization

2s

2p 2p

2s

sp2 Orbital Hybridization

2p

2s

sp2 Orbital Hybridization

Mix together (hybridize) the 2s orbital and two of the three 2p orbitals

2p

2s

sp2 Orbital Hybridization

2 sp2

3 equivalent half-filled sp2 hybrid orbitals plus 1 p orbital left unhybridized

sp2 Orbital Hybridization

sp2 Orbital Hybridization

2 sp2

p

Bonding in Ethylene

2 sp2

the unhybridized p orbital of

carbon is involved in bonding

to the other carbon

p

Bonding in Ethylene Bonding in Ethylene Bonding in Ethylene

2 2 spsp22

pp

each carbon has an unhybridized 2each carbon has an unhybridized 2pp orbital orbital

axis of orbital is perpendicular to the plane of the axis of orbital is perpendicular to the plane of the bonds bonds

Bonding in Ethylene Bonding in Ethylene Bonding in Ethylene

2 2 spsp22

pp

side-by-side overlap of half-filledside-by-side overlap of half-filled

pp orbitals gives a orbitals gives a bondbond

double bond in ethylene has a double bond in ethylene has a

component and a component and a component component

Isomerismo en Alquenos

Isomers are different compounds thathave the same molecular formula.

Isomers

Isomers Isomers

Constitutional isomersConstitutional isomers StereoisomersStereoisomers

different connectivity same connnectivity;different arrangementof atoms in space

Isomers Isomers

Constitutional isomersConstitutional isomers StereoisomersStereoisomers

consider the isomeric alkenes of molecular formula C4H8

2-Methylpropene1-Butene

cis-2-Butene trans-2-Butene

C C

H

H H

CH2CH3

H3C

C C

CH3

H

HH

CH3

C C

H3C

H

C C

H

HH3C

H3C

2-Methylpropene1-Butene

cis-2-Butene

C C

H

H H

CH2CH3

H

CH3

C C

H3C

H

C C

H

HH3C

H3C

Constitutional isomers

2-Methylpropene1-Butene

trans-2-Butene

C C

H

H H

CH2CH3

H3C

C C

CH3

H

H

C C

H

HH3C

H3C

Constitutional isomers

cis-2-Butene trans-2-Butene

H3C

C C

CH3

H

HH

CH3

C C

H3C

H

Stereoisomers

trans (identical or analogous substituents on opposite sides)

Stereochemical Notation

cis (identical or analogous substitutents on same side)

transcis

Interconversion of stereoisomericalkenes does not normally occur.

Requires that component of doublebond be broken.

Figure

transcis

Figure

Naming Steroisomeric Alkenesby the E-Z Notational System

Stereochemical Notation

cis and trans are useful when substituents are identical or analogous (oleic acid has a cis double bond)

cis and trans are ambiguous when analogies are not obvious

C C

CH3(CH2)6CH2 CH2(CH2)6CO2H

H H

Oleic acid

Example

What is needed:

1) systematic body of rules for ranking substituents

2) new set of stereochemical symbols other

than cis and trans

C C

H F

Cl Br

C C

E : higher ranked substituents on opposite sides

Z : higher ranked substituents on same side

higher

lower

The E-Z Notational System

C C

E : higher ranked substituents on opposite sides

Z : higher ranked substituents on same side

higher

lower

The E-Z Notational System

C C

E : higher ranked substituents on opposite sides

Z : higher ranked substituents on same side

Entgegen

higher

higherlower

lower

C C

Zusammen

lower

higher

lower

higher

The E-Z Notational System

C CC C

Answer: They are ranked in order of decreasing atomic number.

Entgegen Zusammen

higher

higherlower

lower

lower

higher

lower

higher

Question: How are substituents ranked?

The E-Z Notational System

The Cahn-Ingold-Prelog (CIP) System

The system that we use was devised byR. S. CahnSir Christopher IngoldVladimir Prelog

Their rules for ranking groups were devised in connection with a different kind of stereochemistry—one that we will discuss later—but have been adapted to alkene stereochemistry.

(1) Higher atomic number outranks lower atomic number

Br > F Cl > H

higher

lower

Br

F

Cl

H

higher

lower

C C

Table CIP Rules

(1) Higher atomic number outranks lower atomic number

Br > F Cl > H

(Z )-1-Bromo-2-chloro-1-fluoroethene

higher

lower

Br

F

Cl

H

higher

lower

C C

Table CIP Rules

(2) When two atoms are identical, compare the atoms attached to them on the basis of their

atomic numbers. Precedence is established

at the first point of difference. —CH2CH3 outranks —CH3

—C(C,H,H)

Table CIP Rules

—C(H,H,H)

(3) Work outward from the point of attachment, comparing all the atoms attached to a particular atom before proceeding furtheralong the chain.

—CH(CH3)2 outranks —CH2CH2OH

—C(C,C,H) —C(C,H,H)

Table CIP Rules

(4) Evaluate substituents one by one. Don't add atomic numbers within groups.

—CH2OH outranks —C(CH3)3

—C(O,H,H) —C(C,C,C)

Table CIP Rules

(5) An atom that is multiply bonded to another atom is considered to be replicated as a

substituent on that atom.

—CH=O outranks —CH2OH

—C(O,O,H) —C(O,H,H)

Table CIP Rules

A table of commonly encountered substituents ranked according to precedence is given on the inside back cover of the text.

Table CIP Rules

Propiedades Físicas de Alquenos

= 0 D

C C

H H

HH

= 0.3 D

H

H H

C C

H3C

Dipole moments

What is direction of dipole moment?

Does a methyl group donate electrons to the double bond, or does it withdraw them?

= 0 D

C C

H H

HH

= 1.4 D

C C

H H

ClH

= 0.3 D

H

H H

C C

H3C

Dipole moments

Chlorine is electronegative and attracts electrons.

= 1.4 D

C C

H H

ClH

= 0.3 D

H

H H

C C

H3C = 1.7 D

H

H Cl

C C

H3C

Dipole moments

Dipole moment of 1-chloropropene is equal to the sum of the dipole moments of vinyl chloride and propene.

= 1.7 D

= 1.4 D

C C

H H

ClH

= 0.3 D

H

H H

C C

H3C

H

H Cl

C C

H3C

Dipole moments

Therefore, a methyl group donates electrons to the double bond.

Alkyl groups stabilize sp2 hybridizedcarbon by releasing electrons

R—C+ H—C+is more stable than

is more stable thanR—C• H—C •

R—C is more stable than H—C

Estabilidades Relativas de Alquenos

Double bonds are classified according tothe number of carbons attached to them.

H

C C

R

H

H

monosubstituted

R'

C C

R

H

H

disubstituted

H

C C

R

H

R'

disubstituted

H

C C

R H

R'

disubstituted

Double bonds are classified according tothe number of carbons attached to them.

R'

C C

R

H

R"

trisubstituted

R'

C C

R

R"'

R"

tetrasubstituted

Electronic

disubstituted alkenes are more stable than monosubstituted alkenes

Steric

trans alkenes are more stable than cis alkenes

Substituent Effects on Alkene Stability

+ 6O2

4CO2 + 8H2O

2700 kJ/mol

2707 kJ/mol

2717 kJ/mol

2710 kJ/mol

Figure Heats of combustion of C4H8

isomers.

alkyl groups stabilize double bonds more than H

more highly substituted double bonds are morestable than less highly substituted ones.

Substituent Effects on Alkene Stability

Electronic

Give the structure or make a molecular model of the most stable C6H12 alkene.

C C

H3C

H3C CH3

CH3

Problem

trans alkenes are more stable than cis alkenes

cis alkenes are destabilized by van der Waalsstrain

Substituent Effects on Alkene Stability

Steric

cis-2-butene trans-2-butene

van der Waals straindue to crowding ofcis-methyl groups

Figure cis and trans-2-Butene

cis-2-butene trans-2-butene

van der Waals straindue to crowding ofcis-methyl groups

Figure cis and trans-2-Butene

Steric effect causes a large difference in stabilitybetween cis and trans-(CH3)3CCH=CHC(CH3)3

cis is 44 kJ/mol less stable than trans

C C

H H

CC CH3

CH3H3C

H3C

H3C CH3

van der Waals Strain

Cicloalquenos

Cyclopropene and cyclobutene have angle strain.

Larger cycloalkenes, such as cyclopenteneand cyclohexene, can incorporate a double bond into the ring with little or no angle strain.

Cycloalkenes

cis-cyclooctene and trans-cycloocteneare stereoisomers

cis-cyclooctene is 39 kJ/ mol more stablethan trans-cyclooctene

Stereoisomeric cycloalkenes

cis-Cyclooctene trans-Cyclooctene

H

H HH

trans-cyclooctene is smallest trans-cycloalkene

that is stable at room temperature

cis stereoisomer is more stable than trans through C11 cycloalkenes

Stereoisomeric cycloalkenes

trans-Cyclooctene

HH

When there are more than 12 carbons in thering, trans-cycloalkenes are more stable than cis.The ring is large enough so the cycloalkene behaves much like a noncyclic one.

Stereoisomeric cycloalkenes

trans-Cyclododecenecis-Cyclododecene

cis and trans-cyclododeceneare approximately equal instability

Structure and Bonding in Alkynes:

sp Hybridization

linear geometry for acetylene

C CH H

120 pm

106 pm 106 pm

C CCH3 H

121 pm

146 pm 106 pm

Structure

Cyclononyne is the smallest cycloalkyne stable enough to be stored at room temperaturefor a reasonable length of time.

Cyclooctyne polymerizeson standing.

Cycloalkynes

C C

Hibridación sp y Enlaces en el Acetileno

C2H2

linear

bond angles: 180°

bond distances: C—H = 106 pm

CC = 120 pm

Structure of Acetylene

HC CH

2s

2p

Promote an electron from the 2s

to the 2p orbital

sp Orbital Hybridization

2s

2p 2p

2s

sp Orbital Hybridization

2p

2s

sp Orbital Hybridization

Mix together (hybridize) the 2s orbital and one of the three 2p orbitals

2p

2s

sp Orbital Hybridization

2 sp

2 equivalent half-filled sp hybrid orbitals plus 2 p orbitals left unhybridized

2 p

sp Orbital Hybridization

sp Orbital Hybridization

2 sp

2 p

Bonding in Acetylene

the unhybridized p orbitals of

carbon are involved in separate

bonds to the other carbon

2 sp

2 p

Bonding in Acetylene Bonding in Acetylene Bonding in Acetylene

one one bond involves one of the p orbitals on each carbon bond involves one of the p orbitals on each carbon

there is a second there is a second bond perpendicular to this one bond perpendicular to this one

2 2 spsp

2 2 pp

Bonding in Acetylene Bonding in Acetylene Bonding in Acetylene

2 2 spsp

2 2 pp

Bonding in Acetylene Bonding in Acetylene Bonding in Acetylene

2 2 spsp

2 2 pp

H C C

Acidity of Acetylene

and Terminal Alkynes

In general, hydrocarbons are exceedingly weak acids

Compound pKa

HF 3.2

H2O 15.7

NH3 36

45

CH4 60

H2C CH2

Acidity of Hydrocarbons

Acetylene is a weak acid, but not nearlyas weak as alkanes or alkenes.

Compound pKa

HF 3.2

H2O 15.7

NH3 36

45

CH4 60

H2C CH2

HC CH 26

Acetylene

Electrons in an orbital with more s character are closer to the

nucleus and more strongly held.

Carbon: Hybridization and Electronegativity

C H H+ +pKa = 60

sp3C :–

H+ +sp2H

C C C C:

–

pKa = 45

H+ + spC C H C C :–

pKa = 26

Objective:

Prepare a solution containing sodium acetylide

Will treatment of acetylene with NaOH be effective?

NaC CH

H2ONaOH + HC CH NaC CH +

Sodium Acetylide

No. Hydroxide is not a strong enough base to deprotonate acetylene.

weaker acidpKa = 26

stronger acidpKa = 15.7

In acid-base reactions, the equilibrium lies tothe side of the weaker acid.

Sodium Acetylide

HO..

.. : HO H..

.. C CH–

H C CH+ + :–

Solution: Use a stronger base. Sodium amideis a stronger base than sodium hydroxide.

NH3NaNH2 + HC CH NaC CH +

Ammonia is a weaker acid than acetylene.The position of equilibrium lies to the right.

–H2N

..: H C CH H

..+ + C CH:

–

stronger acidpKa = 26

weaker acidpKa = 36

H2N

Sodium Acetylide