Estructura de__hidrocarburos
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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