Alkanes
Molecular formula
Alkanes are the simplest hydrocarbons - compounds that contain only carbon and hydrogen. Alkanes contain only single bonds CH and CC bonds. Six of the first alkane compound is:
methane CH4ethane C2H6propane C3H8butane C4H10pentane C5H12hexane C6H14
You can determine the molecular formula of any compound alkanes by using the general formula: CnH2n +2
Isomeri
All alkanes with 4 or more carbon atoms will have isomeri up. This means that there are two or more formula-up that can be made for each molecular formula.
For example, C4H10 could be one of two different molecules of the following:
These molecules are called respectively butane and 2-methylpropane.
Cycloalkanes
Cycloalkanes also contains only CH bonds and CC single bond, only carbon atoms joined in a ring. Smallest cycloalkane is cyclopropane.
If
you count the sheer number of carbon and hydrogen in the picture above,
you will see that the number of C and H atoms are no longer meets the
general formula CnH2n +2. With the incorporation of carbon atoms in a ring, there are two hydrogen atoms missing.
Two hydrogen atoms are missing is no longer needed, because the general formula for a cycloalkane is CnH2n.
Do you think that the molecules formed from the formula is common molecules. All cycloalkanes from cyclopentane upwards exist as "a wrinkled ring".
Cyclohexane for example, has a ring structure that looks like this:
This structure is known as a form of "seat" of cyclohexane - in accordance with the shape a bit like a chair.
Physical Properties
Boiling point
Evidence
Boiling
points are shown in the image above is the boiling point for all
isomers "straight chain" where there is more than one carbon atom.
Note that in the first four alkanes are gases at room temperature. New solid state can be formed starting from the structure of C17H36.
Alkanes
with less than 17 carbon atoms were observed in the solid state is
difficult because each isomer has a melting point and boiling point
different. If there are 17 carbon atoms in the alkane, then it is very much possible isomers formed!
Cycloalkanes have boiling points that are about 10-20 K higher than the comparable straight-chain alkanes.
Explanation-Explanation
Electronegativity difference between carbon and hydrogen is not too large, so there is a very high bond polarity. The molecules themselves have a very small polarity. Even a fully symmetric molecules like methane are not polar at all.
This
means that the only attractive force between the molecules of the
neighboring molecules are Van der Waals dispersion forces. This force is very small for a molecule like methane, but will increase if the molecules grow larger. That is why the boiling point of the alkane increases with molecular size.
The more a branched chain isomer, the boiling point will tend to be lower. Van
der Waals dispersion forces are smaller for molecules shorter chain,
and only affects a very close distance between the molecule with its
neighbors. Molecules with many branches but more difficult short-chain adjacent to each other than the molecules that little has branches.
For example, the boiling point of the three isomers of C5H12 are:
Boiling Point (K)pentane 309.22-methylbutane 301.02.2-dimetilpropana 282.6
Boiling
point slightly higher cycloalkanes may be exacerbated because the
molecules can approach each other due to the ring structure that makes
it more orderly and less "shrink"!
Solubility
Evidence
No different solubility solubility alkanes cycloalkanes.
Alkanes is almost insoluble in water, but soluble in organic solvents. Alkanes in the liquid is a good solvent for many other covalent compounds.
Explanations
Solubility in water
When a molecular substance dissolves in water, the following things happen:
attractive forces between the molecules in a substance is lost. For alkanes, the attractive force is Van der Waals dispersion forces.
attractive forces between the molecules in the water to be lost so that the substance can be mixed with water molecules. In water, an attractive force between the molecules is the main hydrogen bonds.
Energy
required for abolishing intermolecular attractive forces, although the
amount of energy required to remove Van der Waals dispersion forces on
molecules such as methane is very small and can be ignored. However, this does not apply to the hydrogen bonding in water, where it takes a lot of energy to break the hydrogen bonds.
In
other words, a substance will dissolve if there is enough energy
released when new bonds are formed between the substance and the water
to replace the energy used in deciding the initial tensile force.
The only force-pull the newly formed between water molecules are alkanes and Van der Waals force. The
formation of these attractions are not releasing a lot of energy to
replace the energy required to break the hydrogen bonds in water. By him that alkanes are insoluble.
Solubility in organic solvents
In
most organic solvents, the major attractive force between the molecules
of the solvent are Van der Waals force - both dispersion forces and
dipole-dipole force.
This
means that if an alkane dissolved in an organic solvent, the Van der
Waals attractive force is lost and replaced by Van der Waals force a
new one. Termination
of the old-style charm and the formation of a new gravity mutually
eliminate each other in terms of energy - so there is no obstacle for
solubility.
Reactivity kimiawai
Alkanes
Alkanes containing strong CC single bond and CH bonds are also strong. CH
bond polarity so low that there is no molecular ions that carry a
positive or negative number significant to attract other molecules.
By him that alkane-alkane reacts fairly limited.
Some things that can be done on alkanes:
alkanes can be burned, which gutted the entire molecule;
alkanes can be treated with some halogen that break the CH;
alkanes can be broken, namely by breaking CC bonds.
These reactions will be discussed in detail on a separate page (see below).
Cycloalkanes
Sikoalkana have very similar reactivity with alkanes, except for a very small cycloalkanes - particularly cyclopropane. Cyclopropane is much more reactive than you might think.
The reason is because the bond angles in the ring. Normally, when carbon forms four single bonds, the bond angles are approximately 109.5 °. In this corner cyclopropane at 60 °.
With the electron pairs that are close together, occurring repel the electron pairs that connect the carbon atoms. This makes bonds more easily disconnected.
The effect of repelling this will be discussed further on page about the reactions of these compounds with the halogens.
