Estructura y Estereoquímica de Alcanos

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1 Estructura y Estereoquímica de Alcanos
Organic Chemistry, 7th Edition L. G. Wade, Jr. Capítulo 3 Estructura y Estereoquímica de Alcanos ã 2010, Prentice Hall Chapter 3

2 Hidrocarbonos son moléculas que contienen
carbono e hidrógeno a Chapter 3

3 Alcanos Fórmula General: CnH2n+2 Existen en la naturaleza
Alcanos pequeños son gases. CH C2H C3H8 b.p oC oC oC Chapter 3

4 Ejemplo de Alcanos Chapter 3

5 Alcanos Pequeños (CnH2n+2)
Methane Ethane Propane Chapter 3

6 Butano: C4H10 C H n - b u t a no C H C H C H C H i s o - b u t a n o 3
2 n - b u t a no C H 3 C H C H C H i s o - b u t a n o 3 3 Isómeros Constitucionales son compuestos con la misma Fórmula Molecular pero diferente secuencia de enlaces. Chapter 3

7 Pentanos: C5H12 C H 3 2 n-pentano iso-pentano neo-pentano Chapter 3

8 Nomenclatura IUPAC (International Union of Pure and Applied Chemistry)
Nombres comunes para los primeros: methano, etano, propano, butano. Alcanos: sufijo “-ano” después del prefijo relacionado al número de C. Chapter 3

9 Reglas IUPAC Regla 1: Encuentre la cadena más larga y ésta le dará el nombre base del compuesto. Regla 2: Numere la cadena comenzando por el extremo donde más cerca esté el substituyente. Rule 3: Nombre los substituyentes como grupos alquílicos, indicando la posición del substituyente con el número que corresponda. Escriba los substituyentes en orden alfabético. Chapter 3

10 Regla 1: Encuentre la Cadema más Larga
La cadena más larga tiene 6 carbonos: hexano 3-metilhexano Chapter 3

11 Cadena Más Larga Figure: 03_01-04UN.jpg Title: Nomenclature: Same Length Chains Caption: When looking for the longest continuous chain, look to find all the different chains of that length. Often, the longest chain with the most substituents is not obvious. Notes: When there are two carbon chains of the same length, the one that has the most substituents must be chosen to name the compound. Cuando las dos cadenas más largas tengan igual longitud, use la cadena con el mayor número de substituyentes. Chapter 3

12 Nombre de Grupos Alquílicos (substituyentes)
Figure: 03_02.jpg Title: Nomenclature of Alkyl Groups Caption: Some common alkyl groups and their names. Notes: Substituents on a carbon chain are called alkyl groups. They are named by replacing the -ane ending of the alkane with -yl. The “n” and “iso” groupings are used to describe an alkyl chain attached by a primary carbon. The name of an alkyl chain attached by a secondary carbon is “sec,” and that of chains attached by a tertiary carbons “tert.” Chapter 3

13 Regla 2: Numere la Cadena más Larga
3-metilhexano Numere la cadena comenzando con el extremo más cerca a un substituyente. Chapter 3

14 Regla 3: Substituyentes Alquílicos
4-ethyl 2-methyl 4-etil-2-metilhexano Nombre los substituyentes como grupos alquílicos. Indique la posición donde está el substituyente indicando el # del C al cual está enlazado. Escriba los substituyentes alfabéticamente. Chapter 3

15 Dos o Más Substituyentes Iguales
Se usan los prefijos di-, tri-, tetra-, etc. 2,5,7-trimetildecane Chapter 3

16 Solved Problem 3-1 Give the structures of 4-isopropyloctane and 5-t-butyldecane. Solution: 4-Isopropyloctane has a chain of eight carbons, with an isopropyl group on the fourth carbon. 5-t-Butyldecane has a chain of ten carbons, with a t-butyl group on the fifth. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 3

17 Problema De el nombre IUPAC al siguiente compuesto. Chapter 3
Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 3

18 Solved Problem 3-2: Solution
The longest carbon chain contains eight carbon atoms, so this compound is named as an octane. Numbering from left to right gives the first branch on C2; numbering from right to left gives the first branch on C3, so we number from left to right. Copyright © 2006 Pearson Prentice Hall, Inc. 4-isopropyl-2,2,3,6-tetramethyloctane Chapter 3

19 Puntos de Ebullición de Alcanos
El p. eb. aumenta a medida que aumenta el # de C, debido al aumento en la superficie de contacto. Figure: 03_03.jpg Title: Boiling Points of Branched and Unbranched Alkanes Caption: Alkane boiling points. Comparison of the boiling points of the unbranched alkanes (blue) with those of some branched alkanes (red). Because of their smaller surface areas, branched alkanes have lower boiling points than unbranched alkanes. Notes: The only intermolecular force of nonpolar molecules are London dispersion forces which result from induced dipole attractions. Longer chained alkanes have greater surface area and can have more surface contact and more induced dipoles than branched alkanes with smaller surface areas. Chapter 3

20 Puntos de Fusión de Alcanos
Los p.f. aumentan con el #C de la cadena. Alcanos con # par de C tienen p.f. mayor que aquellos con #C impar. Alcanos ramificados tienen p.f. mayor que los no ramificados. Figure: 03_04.jpg Title: Melting Points of Alkanes Caption: Alkane melting points. The melting point curve for n-alkanes with even numbers of carbon atoms is slightly higher than that for alkanes with odd numbers of carbons. Notes: In solids, the packing of the molecules into a three dimensional structure affects the melting point. When molecules can pack in neat order avoiding empty pockets the melting point will be higher than when the packing is not ordered. Alkanes with an even number of carbons pack better than those with an odd number of carbons. Chapter 3

21 Cracking e Hidrocracking
Chapter 3

22 Metano Tetraédrico C sp3 con ángulos de 109.5º. Chapter 3
Figure: 03_04-08UN.jpg Title: Methane Representations Caption: The simplest alkane is methane, CH4. Methane is perfectly tetrahedral, with the 109.5° bond angles predicted for an sp3 hybrid carbon. The four hydrogen atoms are covalently bonded to the central carbon atom, with bond lengths of 1.09Å. Notes: Any carbon with four sigma bonds has an sp3 hybridization. Tetraédrico C sp3 con ángulos de 109.5º. Chapter 3

23 Etano Dos C sp3 . Rotación libre alrededor del enlace σ C—C.
Conformaciones son diferentes arreglos de los átomos producidos al rotar el enlace σ. Figure: 03_04-09UN.jpg Title: Ethane Caption: Ethane, the two-carbon alkane, is composed of two methyl groups with overlapping sp3 hybrid orbitals forming a sigma bond between them. Notes: Ethane has two sp3 carbons. The C-C bond distance is 1.54Å and there is free rotation along this bond. Chapter 3

24 Conformaciones de Etano
Figure: 03_04-10UN.jpg Title: Conformations of Ethane Caption: The different arrangement formed by rotations about a single bond are called conformations, and a specific is called conformer. Pure conformers cannot be isolated in most cases, because the molecules are constantly rotating through all the possible conformations. Notes: Some conformations can be more stable than others. Los confórmeros puros no pueden ser aislados porque la rotación es constante y rápida. Chapter 3

25 Proyecciones Newman Chapter 3 Figure: 03_05.jpg Title:
Newman Projections Caption: The Newman projection looks straight down the carbon-carbon bond. Notes: The Newman projection is the best way to judge the stability of the different conformations of a molecule. Chapter 3

26 Conformaciones de Etano
Figure: 03_07.jpg Title: Conformational Analysis of Ethane Caption: The torsional energy of ethane is lowest in the staggered conformation. The eclipsed conformation is about 3.0 kcal/mol (12.6 kJ/mol) higher in energy. At room temperature, this barrier is easily overcome, and the molecules rotate constantly. Notes: The staggered conformations are lower in energy than the eclipsed conformation because the staggering allows the electron clouds of the C-H bonds to be as far apart as possible. The energy difference is only 3 kcals/mol which can be easily overcome at room temperature. La Energía de Torsión de etano es más baja en su conformación alternada. La eclipsada es 3.0 kcal/mol (12.6 kJ/mol) más alta. Chapter 3

27 Conformaciones de Propano
Figure: 03_08.jpg Title: The Newman Projection of Propane Caption: Propane is shown here as a perspective drawing and as a Newman projection looking down one of the carbon-carbon bonds. Notes: Chapter 3

28 Conformaciones de Propano
Figure: 03_09.jpg Title: Conformational Analysis of Propane Caption: Torsional energy of propane. When a bond of propane rotates, the torsional energy varies much like it does in ethane, but with 0.3 kcal/mol (1.2 kJ/mol) of additional torsional energy in the eclipsed conformation. Notes: Much like ethane the staggered conformations of propane is lower in energy than the eclipsed conformations. Since the methyl group occupies more space than a hydrogen, the torsional strain will be 0.3 kcal/mol higher for propane than for ethane. La conformación alternada tiene menos energía (más estable) que la eclipsada. La Energía Torsional es 3.3kcal/mol (0.3 kcal/mol más que en etano). Chapter 3

29 Conformaciones de Butano
Figure: 03_10.jpg Title: Newman Projections of Butane Caption: Butane conformations. Rotations about the center bond in butane give different molecular shapes. Three of these conformations are given specific names. Notes: For butane there will be two different staggered conformations: gauche and anti. The gauche conformation has a dihedral angle of 60° between the methyl groups while the anti conformation has a dihedral angle of 180° between the methyl groups. There distinct eclipsed conformation when the dihedral angle between the methyl groups is 0°, this conformation is referred to as totally eclipsed. Butano tiene 2 conformaciones alternadas diferentes: gauche (60° entre los grupos metilos) y anti (180° entre los grupos metilos). Chapter 3

30 Chapter 3 Figure: 03_11.jpg Title: Conformational Analysis of Butane
Caption: Torsional energy of butane. The anti conformation is lowest in energy, and the totally eclipsed conformation is highest in energy. Notes: The eclipsed conformations are higher in energy than the staggered conformations of butane, especially the totally eclipsed conformation. Among the staggered conformations, the anti is lower in energy because it has the electron clouds of the methyl groups as far apart as possible. Chapter 3

31 Tensión Estérica La conformación totalmente eclipsada es mayor en energía porque los dos grupos metilos están tan cerca que la repulsión de sus nubes electrónicas es grande. Esta interferencia entre dos grupo voluminosos se llama tensión estérica o impedimento estérico. Figure: 03_11-01UN.jpg Title: Totally Eclipsed Conformation of Butane Caption: The totally eclipsed conformation is about 1.4 kcal (5.9 kJ) higher in energy than the other eclipsed conformations, because it forces the two end methyl groups so close together that their electron clouds experience a strong repulsion. This kind of interference between two bulky groups is called steric strain or steric hindrance. Notes: The other eclipsed conformations are lower in energy than the totally eclipsed conformation but are still more unstable than the staggered conformations. Chapter 3

32 Cicloalcanos CnH2n Chapter 3 Figure: 03_12.jpg Title: Cycloalkanes
Caption: Structures of some cycloalkanes. Notes: The molecular formula of alkanes is CnH2n, two hydrogen less than an open chain alkane. Their physical properties resemble those of alkanes. Chapter 3

33 Propiedades Físicas No polares Relativamente inertes
p.eb. y p.f. dependen de su peso molecular. Chapter 3

34 Nomenclatura El anillo es la cadena principal: los grupos enlazados al anillo se nombrarán como susbstituyentes. Si hay un sólo grupo no es necesaria la numeración. etilciclopentano Chapter 3

35 Nomenclatura Si hay dos o más substituyentes se enumerará la cadena de manera tal que se asigne a los substituyentes los números más bajos. 1 1 3 3 1,3-dimetilciclohexano 3-etil-1,1-dimetilciclohexano Chapter 3

36 Geometric Isomers Same side: cis- Opposite side: trans-
1 Same side: cis- cis-1,2-dimethylcyclohexane 2 2 Opposite side: trans- 1 trans-1-ethyl-2-methylcyclohexane Chapter 3

37 Estabilidad de Cicloalcanos
Los anillos más comunes en la naturaleza son de 5 y 6 miembros. Los C de los cicloalcanos son sp3 y, por lo tanto, requieren ángulos de º. Si esto no fuese el caso aparecería la Tensión angular. Chapter 3

38 Cyclopropano: C3H6 Los ángulos están comprimidos a 60°.
Existe tensión angular. No hay interacción lineal entre los orbitales sp3 (“banana bonds”). Figure: 03_15.jpg Title: Angle Strain in Cyclopropane Caption: Angle strain in cyclopropane. The bond angles are compressed to 60° from the usual 109.5° bond angle of sp3 hybridized carbon atoms. This severe angle strain leads to nonlinear overlap of the sp3 orbitals and “bent bonds.” Notes: The angle compression of cyclopropane is 49.5°. The high reactivity of cyclopropanes is due to the non-linear overlap of the sp3 orbitals. Chapter 3

39 Tensión Estérica Tensión angular y Tensión estérica Chapter 3
Figure: 03_16.jpg Title: Conformations of Cyclopropane Caption: Torsional strain in cyclopropane. All the carbon-carbon bonds are eclipsed, generating torsional strain that contributes to the total ring strain. Notes: The angle strain and the torsional strain in cyclopropane make this ring size extremely reactive. Tensión angular y Tensión estérica Chapter 3

40 Cyclobutano: C4H8 Figure: 03_14.jpg Title: Ring Strain and Torsional Strain of Cyclobutane Caption: The ring strain of a planar cyclobutane results from two factors: Angle strain from the compressing of the bond angles to 90° rather than the tetrahedral angle of 109.5°, and torsional strain from eclipsing of the bonds. Notes: The angle compression for butane is 19.5°. Angle strain and torsional strain account for the high reactivity of 4-membered rings. La Tensión Anular proviene de dos factores: Tensión Angular (109.5o vs 90o) y Tensión Estérica (hidrógenos eclipsados). Chapter 3

41 Ciclobutano no Planar Adopta una conformación no planar para liberar Tensión Anular. Ciclobutano adopta la conformación “envelope”. Figure: 03_17.jpg Title: Conformations of Cyclobutane Caption: The conformation of cyclobutane is slightly folded. Folding gives partial relief from the eclipsing of bonds, as shown in the Newman projection. Compare this actual structure with the hypothetical planar structure in Figure 3-14. Notes: Cyclic compound with 4 carbons or more adopt non-planar conformations to relieve ring strain. Cyclobutane adopts the folded conformation to decrease the torsional strain caused by eclipsing hydrogens. Chapter 3

42 Ciclopentano: C5H10 Figure: 03_18.jpg Title: Conformations of Cyclopentane Caption: The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent CH2 groups. Notes: To relieve ring strain, cyclopentane adopts the envelope conformation. Nuevamente, la conformación adoptada es como un sobre. Chapter 3

43 Conformación Silla de Ciclohexano
Figure: 03_19.jpg Title: Conformations of Cyclohexane Caption: The chair conformation of cyclohexane has one methylene group puckered upward and another puckered downward. Viewed from the Newman projection, the chair conformation has no eclipsing of the carbon-carbon bonds. The bond angles are 109.5°. Notes: Cyclohexane can adopt four non-planar conformations: chair, boat, twist boat, and half-chair. The most stable conformation is the chair because it has all the C-H bonds staggered. Chapter 3

44 Conformación Bote de Ciclohexano
Figure: 03_20.jpg Title: Boat Conformation of Cyclohexane Caption: In the symmetrical boat conformation of cyclohexane, eclipsing of bonds results in torsional strain. In the actual molecule, the boat conformation is skewed to give the twist boat, a conformation with less eclipsing of bonds and less interference between the two flagpole hydrogens. Notes: In the boat conformation all bonds are staggered except for the “flagpole” hydrogens. There is steric hindrance between these hydrogens so the molecule twists a little producing the twist boat conformation which is 1.4 kcal (6 kJ) lower in energy than the boat. Chapter 3

45 Diagrama de Energía Conformacional de Ciclohexano
Figure: 03_21.jpg Title: Conformational Energy Diagram of Cyclohexane Caption: Conformational energy of cyclohexane. The chair conformation is most stable, followed by the twist boat. To convert between these two conformations, the molecule must pass through the unstable half-chair conformation. Notes: Interconversion between chair conformations require that cyclohexane go through its higher energy conformations. Chapter 3

46 Posiciones Axiales y Ecuatoriales
Figure: 03_22.jpg Title: Chair Conformation of Cyclohexane Caption: The axial bonds are directed vertically, parallel to the axis of the ring. The equatorial bonds are directed outward, toward the equator of the ring. As they are numbered here, the odd-numbered carbons have their upward bonds axial and their downward bonds equatorial. The even-numbered carbons have their downward bonds axial and their upward bonds equatorial. Notes: All the C-H bonds are staggered in the chair conformation. Axial hydrogens are pointed straight up or down, parallel to the axis of the ring. Equatorial hydrogens, like their name suggests, are pointed out of the ring along the “equator” of the molecule. Chapter 3

47 Interconversión Silla-Silla
Figure: 03_23.jpg Title: Chair-Chair Interconversion Caption: Chair-chair interconversion of methylcyclohexane. The methyl group is axial in one conformation, and equatorial in the other. Notes: La interconversión de una silla en otra produce que los substituyentes Axiales pasen a ser ecuatoriales en la otra silla. Chapter 3

48 Metil Axial en Metilciclohexano
Figure: 03_24.jpg Title: Newman Projection of Methylcyclohexane: Methyl Axial Caption: (a) When the methyl substituent is in an axial position on C1, it is gauche to C3. (b) The axial methyl group on C1 is also gauche to C5 of the ring. Notes: In the Newman projection it is easier to see the steric interaction between the methyl substituent and the hydrogens and carbons of the ring. Chapter 3

49 Metilo Ecuatorial Chapter 3 Figure: 03_25.jpg Title:
Newman Projection of Methylcyclohexane: Methyl Equatorial Caption: Looking down the C1-C2 bond of the equatorial conformation, we find that the methyl group is anti to C3. Notes: An equatorial methyl group will be anti to the C3. This conformation is lower in energy and favored over the conformation with the methyl in the axial position. Chapter 3

50 Interacciones 1,3-Diaxial
Figure: 03_26.jpg Title: 1,3-Diaxial Interaction Caption: The axial substituent interferes with the axial hydrogens on C3 and C5. This interference is called a 1,3-diaxial interaction. Notes: Los substituyentes axiales interfieren con los H axiales del C(3) y C(5). Esta interferencia es llamada interacciones 1,3-diaxial. Chapter 3

51 Cis-1,3-dimetilciclohexano
Figure: 03_26-01UN.jpg Title: Chair Conformations of cis-1,3-Dimethylcyclohexane Caption: Two chair conformations are possible for cis-1,3-dimethylcyclohexane. The unfavorable conformation has both methyl groups in axial positions, with a 1,3-diaxial interaction between them. The more stable conformation has both methyl groups in equatorial positions. Notes: Alkyl substituents on cyclohexane rings will tend to be equatorial to avoid 1,3-diaxial interactions. cis-1,3-dimethylcyclohexane can have both methyl groups on axial positions but the conformation with both methyls in equatorial positions is favored. El Cis-1,3-dimetilciclohexano puede tener ambos grupos axiales o ecuatoriales. La conformación en que ambos están ecuatoriales es la más estable. Chapter 3

52 Trans-1,3-dimetilciclohexano
Figure: 03_26-02UN.jpg Title: Chair Conformations of trans-1,3-Dimethylcyclohexane Caption: Either of the chair conformations of trans-1,3-dimethylcyclohexane has one methyl group in an axial position and one in an equatorial position. These conformations have equal energies, and they are present in equal amounts. Notes: Alkyl substituents on cyclohexane rings will tend to be equatorial to avoid 1,3-diaxial interactions. trans-1,3-dimethylcyclohexane has one methyl group axial and the other equatorial. Chair interconversion would still produce an axial and an equatorial methyl. In this case both chairs have the same energy, and they are present in equal amounts. Ambas conformaciones tienen un metil axial y otro ecuatorial. Ambas tienen la misma energía. Chapter 3

53 Decalín Figure: 03_27.jpg Title: cis- and trans-Decalin Caption: cis-Decalin has a ring fusion where the second ring is attached by two cis bonds. trans-Decalin is fused using two trans bonds. The six-membered rings in cis- and trans-decalin assume chair conformations. Notes: The correct IUPAC name for decalin is bicyclo[3.3.0]decane but it is commonly known as decalin. There are two possible geometric isomers for decalin: cis and trans. Cis-decalín tiene dos anillos fundidos donde el segundo anillo está enlazado al primero por dos enlaces cis. EnTrans-decalín el segundo anillo está unido por dos enlaces trans. ElTrans-decalin es más estable porque los grupos alquílicos están ecuatoriales. Chapter 3

54 Tert-butilciclohexano
Figure: 03_27-11UN.jpg Title: Conformations with Extremely Bulky Groups Caption: Some groups are so bulky that they are extremely hindered in axial positions. Cyclohexanes with tertiary-butyl substituents show that an axial t-butyl group is severely hindered. Regardless of the other groups present, the most stable conformation has a t-butyl group in an equatorial position. The following figure shows the severe steric interactions in a chair conformation with a t-butyl group axial. Notes: Alkyl substituents on cyclohexane rings will tend to be equatorial to avoid 1,3-diaxial interactions. Groups like tert-butyl are so bulky that it will force the chair conformation where it is in the equatorial position, regardless of other groups present. Chapter 3

55 Solved Problem 3-3 (a) Draw both chair conformations of cis-1, dimethylcyclohexane, and determine which conformer is more stable. (b) Repeat for the trans isomer. (c) Predict which isomer (cis or trans) is more stable. Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 3

56 Solved Problem 3-1 Draw the most stable conformation of trans-1-ethyl-3-methylcyclohexane. Solution: Copyright © 2006 Pearson Prentice Hall, Inc. Chapter 3