Why do alkanes have no functional groups


2. Organic compounds - classification according to functional groups.
Alkanes: molecules without functional groups

2.1 Functional groups

There are approximately nine million known organic substances today. Fortunately, these connections can be due to their functional groups be classified in substance classes. The functional groups determine the properties and reactivity of the entire molecule, and chemical reactions take place almost exclusively on them. E.g. The C = C double bonds in ethylene and & # 945-pinene react with bromine in a similar way:

A list of the most common functional groups is enclosed separately. For example, they can be divided into the following categories:

Functional groups with C-C multiple bonds

A single bond between C and an electronegative atom

Functional groups with a C = O double bond (carbonyl group)

2.2 Alkanes (molecules without functional groups): nomenclature, isomerism, conformation.

Molecules whose empirical formula has the general form C.xHy is referred to as Hydrocarbons. Hydrocarbons that contain only single bonds have the designation Alkanes or saturated hydrocarbons.

Due to their structure, alkanes can be divided into different types: the straight chain Alkanes, the branched Alkanes in which there are one or more branch points in the carbon chain, and the cyclic alkanes or Cycloalkanes as well as the more complicated bicyclic, tricyclic, and polycyclic alkanes.

3D view

Molecules that have the same molecular formula (sum formula) but have different chemical or physical properties are called Isomers. e.g. there are two C4H10 Molecules and three possible C.5H10 Molecules. With the empirical formula C30H62there are 4,111,846,763 possible isomers.

A distinction is made between two types of isomerism: Constitutional isomerism and Stereoisomerism. Both can be found in all organic material classes (stereoisomerism will be discussed later).

Constitutional isomers have the same empirical formula, but they differ in the way their atoms and groups are linked. So they differ in theirs Constitutions (Atomic links, connectivities): e.g.

3D view

The first four alkanes have their own names that have been included in the IUPAC (International Union of Pure and Applied Chemistry) system, but they all end with -at :

Names of the straight chain alkanes CnH2n + 2 ;

With the exception of the first four, the names of the alkanes are derived from the Greek numeral for the number of carbon atoms and the ending -at together.

nSurnameformulanSurnameformula
1methaneCH411UndecaneCH3(CH2)9CH3
2EthaneCH3CH312DodecaneCH3(CH2)10CH3
3propaneCH3CH2CH313TridecaneCH3(CH2)11CH3
4butaneCH3CH2CH2CH314TetradecaneCH3(CH2)12CH3
5PentaneCH3(CH2)3CH315PentadecaneCH3(CH2)13CH3
6HexaneCH3(CH2)4CH316HexadecaneCH3(CH2)14CH3
7HeptaneCH3(CH2)5CH317HeptadecaneCH3(CH2)15CH3
8OctaneCH3(CH2)6CH318OctadecaneCH3(CH2)16CH3
9NonaneCH3(CH2)7CH319NonadecaneCH3(CH2)17CH3
10DecaneCH3(CH2)8CH320EicosanCH3(CH2)18CH3

The group (CH3, C2H5, C3H7....) has the ending -yl (Methyl, ethyl, ...).

If you want to name branched alkanes, you have to follow the IUPAC nomenclature rules (briefly described in the textbook by Fox & Whitesell p.22).

i) Find the longest chain of carbon atoms in the molecule and name it

If there are two chains of the same length, the chain with more substituents takes precedence.

ii) Determine the names of the alkyl groups attached to the longest chain

iii) Number the carbon atoms of the longest chain from the end closest to a substitution.

and in the case of 2 substituents with the same distance from the chain end then alphabetically.

iv) Write the name of the alkane by first arranging the names of the side chains in alphabetical order (each is preceded by the number of the carbon atom to which it is attached and a hyphen), then adding the name of the parent alkane, as on Edge shown, add.

2.3 Physical properties of the alkanes

What do the three-dimensional structures of the alkanes look like and what physical properties do they have?

At room temperature the homologous alkanes with a smaller molar mass are gases or colorless liquids, while those with a larger molar mass are solids. e.g .:

Melting points, boiling points and density ([D204] at 20 OC based on water of 4 OC) as a function of the molecular size of straight-chain alkanes

2.4 Chemical properties

At room temperature, the alkanes are practically completely inert to most reagents. If a mixture of an alkane with chlorine or with bromine is exposed to vigorous light, halogenation takes place.

The combustion of the alkanes to CO2 and H2O is strongly exothermic:

The largest natural sources of alkanes are in petroleum. In the US, around 99% of all organic raw materials come from petroleum:

2.5 conformation

Structures that can be converted into one another by rotating around single bonds are referred to as Conformations.

For the molecule of ethane, whose C-C bond is a rotationally symmetrical bond, one would expect that free rotation about the C-C bond is possible. But experimental investigations and calculated thermodynamic data only agree if an energy barrier of around 12.6 KJ / mol (3.0 Kcal / mol) is assumed for the rotation, which must correspond to the energy difference between the staggered and ecliptic conformation:

Sawhorse projection

Rotation around the C-C bond

Newman projection

Potential energy of the conformations of ethane as a function of the torsion angle:

Molecules that exist in conformations that correspond to the energy minima are called Conformers (or conformational isomers).

While at propane the energy difference between the staggered and ecliptic conformation is only slightly increased compared to ethane (14.6 KJ / mol) despite the presence of a further methyl group instead of a hydrogen atom n-butane because of the greater interactions between two methyl groups, the different conformations are energetically stronger from one another:

With higher alkanes, of course, many more different excellent conformations are possible than with ethane or n-butane. However, the differences in energy between them are also only slight, so that the conformers cannot be grasped as substances. In the solid state, only zigzag chains occur most frequently, with the hydrogen atoms throughout anti-periplanar Position to each other:

2.6 Cyclic alkanes

The nomenclature of this class of compounds is very simple: the name of the open-chain alkane with the same number of carbon atoms is simply the prefix cyclo prefixed:

If one examines molecular models of disubstituted cycloalkanes in which both substituents are on different carbon atoms, one sees that there are two possible isomers in each case. e.g.

These are not constitutional isomers, but Stereoisomers. Stereoisomers have the same constitution but different geometries and topographies (i.e. they differ in the spatial arrangement of the bound atoms or groups).

2.7 The structure of the cycloalkanes

Cyclopropane has the shape of a flat equilateral triangle, the C-C-C angles are therefore 60Owhich is a considerable deviation from 109.5O means. In addition, all hydrogen atoms are eclipsed to one another. Cyclopropane is far more unstable than one would expect for a molecule with three methylene groups:

 

All three C-C bonds are bent (orbital angle 104O), although the overlap is large enough to result in a bond. The bond dissociation energies (C-C) = 272 KJ / mol (65 Kcal / mol) are relatively weak (more reactive!).

The structure of Cyclobutane shows that the molecule is not flat but folded. The tension created by the eight eclipsed hydrogen atoms is partially reduced as a result:

 

One might expect that Cyclopentane is built flat because the angles are in a regular pentagon 108O be. However, such a planar arrangement would be associated with ten unfavorable ecliptic H-H interactions. This is avoided by folding the ring (envelope "envelope" structure):

The Cyclohexane ring is one of the most common and important structural units in organic chemistry. A hypothetical planar cyclohexane contained twelve ecliptic H-H interactions and six times the angular stress. However, there is an almost tension-free conformation of cyclohexane:

a chair conformation

Methylene groups are as gauche-Substituents of the adjacent C-C bond. A anti-Compliant is not possible in a six-membered ring:

Six C-H bonds are parallel to the axis of rotation of the molecule, and are called axial designated. The other six are perpendicular to that axis - they will equatorial called :

There are other (less stable) conformations of cyclohexane. One of them is that Tub (boat) shape:

It is ≈ 30 KJ / mol more energetic than the chair shape. If one of the C-C bonds of the ring is rotated relative to the neighboring one, the conformation stabilizes somewhat because the interactions between the inner hydrogen atoms are canceled. This new conformation is known as Twist shape (≈ 23 KJ / mol more energetic than the chair shape).

Cyclohexane is not a rigid structure! There is only one isomer of methyl- or bromo-cyclohexane (not two: axial vs. equatorial).

One chair conformation changes into the other, whereby axial and equatorial hydrogen atoms exchange their positions, i.e. that at Folding the ring down all axial hydrogen atoms become equatorial and vice versa:

The activation energy for this process is 45.2 KJ / mol (small enough that the two chair shapes rearrange one another extremely quickly at room temperature).

The following animation shows the potential energy and conformational change of the cyclohexane molecule during the flipping process. With the help of the control fields (bottom left) the animation can be stopped or individual conformations can be viewed separately.

(© Ian Hunt, University of Calgary)

In methylcyclohexane, the methyl group can take either an equatorial or an axial position:

In the equatorial conformer, the methyl group protrudes into the space in which there are no other parts of the molecule, while in the axial conformer the methyl substituent gauche is to two C-C bonds and very close to two axial hydrogen atoms on the same side of the ring; referred to as 1,3-diaxial interactions.

 

Both chair forms of methylcyclohexane are in equilibrium with each other, with the equatorial conformer being favored in a ratio of 95: 5. The free enthalpy difference, & # 916GO, between the axial and the equatorial isomer has been measured for many monosubstituted cyclohexanes (see table below). In many cases (but not in all) the energy difference between the two forms increases with the size of the substituent. With tBu-Cyclohexane the difference in energy is so big (& # 916GO = -RT lnK) that only a small proportion (0.01%) of the axial isomer is in solution (Der tert-Bu substituent fixes the conformation).

Table: Differences in the free enthalpy between axial and equatorial conformers of cyclohexane (in all examples the equatorial form is more stable)
Substituent& # 916GO/ (kJ / mol)Substituent& # 916GO/ (kJ / mol)
H-0F-1.05
CH3-7.12Cl-2.18
CH3CH2-7.33Br-2.30
(CH3)2CH-9.21I-1.93
(CH3)3C-~38HO-3.94
-COOH5.90CH3O-3.14
-COOMe5.40H2N-5.86

2.8 Poly-substituted cyclohexanes

In the case of disubstituted cyclohexanes, the conformation with the largest number of equatorial substituents is generally preferred, e.g .:

1,2-dimethylcyclohexane

1,3-dimethylcyclohexane

and 1,4-dimethylcyclohexane?

Let us now consider a cyclohexane with two different substituents - cis- and trans-1-Bromo-3-Methylcyclohexane: The Most Stable Conformations?

2.9 Polycyclic alkanes

in the Decalin-Molecule, two cyclohexane rings share two carbon atoms with each other, both rings are condensed or fused ;

Steroids are common in nature, and a number of steroids act as Hormones. e.g .: