Sabtu, 01 Agustus 2009

HYDROCARBON COMPOUNDS

THE CHARACTERISTIC OF CARBON ATOM
Atomic number of carbon atom is 6, so the electronic configuration is (2,4).
Several factors make carbon essential to life.
1· The ease with which carbon atoms form bonds to other carbon atoms.
The strength of C C single bonds and the covalent bonds carbon forms to other non metals, such as N, O, P, and S.
2· The ability of carbon to form multiple bonds to other nonmetals, including C, N, O, P, and S atoms.
A few structural characteristics of the carbon atom.
Carbon is tetracovalent. That means that a carbon atom typically makes four bonds to other atoms and that these bonds are covalent--formed by sharing an electron pair between the two atoms joined by the bond. Such arrangements provide eight valence electrons for a carbon atom, so that it's electronic configuration is like that of the very stable noble gas neon. Similarly, hydrogen forms one covalent bond, oxygen two, and nitrogen three.
Carbon can form multiple covalent bonds. That is, a single carbon atom can form a double (to C, O or N) or triple (to C or N) bond to another atom. A double bond would involve two electron pairs between the bonded atoms and a triple bond would involve three electron pairs.
Bonds between carbon and atoms other than carbon or hydrogen are polar. That is, in a bond between carbon and oxygen or nitrogen the electrons are closer to the more electronegative element (oxygen or nitrogen) than to the carbon, so the carbon has a slightly positive charge. (Fluorine is the most electronegative element, and the elements close to fluorine in the periodic table are also quite electronegative.)
Bonds between one carbon atom and another and between a carbon and a hydrogen are non polar. That is, the electron pair forming the bond is quite evenly shared by the atoms.
We can predict the geometry of the bonds around an atom by using the idea that electron pairs and groups of electron pairs (such as in double or triple bonds) repel each other (Valence Shell Electron Pair Repulsion--VSEPR--Theory).
ALKANES AND CYCLOALKANES
Compounds that contain only carbon and hydrogen are known as hydrocarbons. Those that contain as many hydrogen atoms as possible are said to be saturated. The saturated hydrocar-bons are also known as alkanes.
The Nomenclature of Alkanes
Naming branched alkanes. The nomenclature becomes more complex if the alkane branches. In such a case, there are several rules that you must follow to give the alkane the correct name.
1. Find the longest chain of carbons in the molecule. The number of carbons in the longest chain becomes the parent name (refer to the above table)
2. After finding the parent chain, you number the parent chain starting with the end nearest the first substituent (a substituent is any fragment that juts off the main chain).
3. Next, determine the names of all substituents. Substituents are named as if the piece were a separate molecule, except that the suffix of yl is used rather than ane. Thus, a two-carbon substituent would be an ethyl substituent (not an ethane substituent).
4. Put the substituents in alphabetical order (ie. ethyl before methyl) in front of the parent name.

ALKENES AND ALKYNES
Alkenes are examples of unsaturated hydrocarbons because they have fewer hydrogen atoms than the corresponding alkanes.
The IUPAC nomenclature for alkenes names these compounds as derivatives of the parent alkanes. The presence of the C=C double bond is indicated by changing the -ane ending on the name of the parent alkane to -ene.
The location of the C=C double bond in the skeleton structure of the compound is indicated by specifying the number of the carbon atom at which the C=C bond starts.
Compounds that contain C C triple bonds are called alkynes. These compounds have four less hydrogen atoms than the parent alkanes, so the generic formula for an alkyne with a single C C triple bond is CnH2n-2. The simplest alkyne has the formula C2H2 and is known by the common name acetylene.
The IUPAC nomenclature for alkynes names these compounds as derivatives of the parent alkane, with the ending -yne replacing -ane.
In addition to compounds that contain one double bond (alkenes) or one triple bond (alkynes), we can also envision compounds with two double bonds (dienes), three double bonds (trienes), or a combination of double and triple bonds.
ISOMERISM
Structural Isomerism
This isomerism shows that two or more compounds which have the same molecular formulas, have different structural formulas. For example, n-butane and isobutane (2-methyl propane).
Structural isomers have similar chemical properties but different physical properties. n-Butane melts at -138.4C and boils at -0.5C, whereas isobutane melts at -159.6C and boils at -11.7C .
There are 5 steps for drawing isomers:
1. Draw the main chain.
2. Draw the main chain minus 1 carbon, and add a methyl group to as many positions as possible. Never add the methyl groups to the end of the chain, and watch not to repeat structures (it's okay if you accidentally repeat structures, for they will be caught and discarded when you do step 5).
3. Draw the main chain minus 2 carbons, and add two one-carbon groups (two methyls) or one 2-carbon group (an ethyl) to as many positions possible, trying not to repeat structures.
4. Continue subtracting and adding groups in this fashion until you run out of carbons or doing so only results in repeated structures.
Give the IUPAC name to all the compounds you drew. If you accidentally drew the same one twice, they will have identical names, and you can cross one of them off.

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