Organic compound is a compound in which one or more atoms of carbon are covalently linked to atoms of other elements, most commonly hydrogen, oxygen, or nitrogen.
The Carbon Atom
Carbon is a nonmetallic tetravalent element making four electrons available to form covalent chemical bonds. Carbon is the only element that can form so many different compounds because each carbon atom can form four chemical bonds to other atoms. This makes carbon unique among the other elements.
Reasons for having many carbon compounds:
• carbon has the ability to form long carbon-to-carbon chains
• carbon atoms can bind to each other not only in straight chains but in complex branchings
• carbon atoms can share electrons to form single, double or triple bond
• with same collection of carbon atoms and bonds, isomers can be formed
• all of the electrons that are not being used to bond carbon atoms together into chains and rings can be used to form bonds with atoms of several other elements
Bonding Patterns in Hydrocarbons
Hydrocarbons are molecules that contain only carbon and hydrogen. Due to carbon's unique bonding patterns, hydrocarbons can have single, double, or triple bonds between the carbon atoms. The bonding of hydrocarbons allows them to form rings or chains.
Hydrocarbons are classified as either aliphatic or aromatic . Aliphatic described hydrocarbons derived by chemical degradation of fats or oils. Aromatic hydrocarbons described hydrocarbons derived by chemical degradation of certain pleasant-smelling plant extracts. Aliphatic hydrocarbons are divided into alkanes, alkenes, and alkynes. Alkanes have only single bonds, alkenes contain a carbon-carbon double bond, and alkynes contain a carbon-carbon triple bond. Aromatic hydrocarbons are those that are significantly more stable than their Lewis structures would suggest; i.e., they possess “special stability.” They are classified as either arenes, which contain a benzene ring as a structural unit, or nonbenzenoid aromatic hydrocarbons, which possess special stability but lack a benzene ring as a structural unit.
Illustration: H H
Alkane : Ethane H-C – C- H with single bond between C atoms
H H
Alkene: Ethene H H with double bond between C atoms
C=C
H H
Alkyne: Ethyne H – C = C- H with triple bond between C toms
Naming of Hydrocarbon
Some Prefixes Used in Naming Aliphatic Hydrocarbon:
Number of C Prefix Number of C Prefix
1 meth- 6 hex-
2 eth- 7 hept-
3 prop- 8 oct-
4 but- 9 non-
5 pent- 10 dec-
Naming of Hydrocarbon
I. Alkane: prefix + ane
Example : 5-carbon alkane : pentane
II. Alkene: prefix + ene
Example: 5-carbon alkene: pentene
III.Alkyne: prefix + yne
Example: 5-carbon alkyne: pentyne
Naming of of Cycloalkanne
Example 1: cyclo + name of alkane
H H
C Name: cyclopropane
H - C C - H
H H
Alkyl
It is an alkane minus one hydrogen. In naming alkyl, replace -ane with –yl of the alkane’s name.
Example: alkane alkane structure alkyl alkyl structure
H H
methane H – C – H methyl H – C - H H
IUPAC Rules for Alkane Nomenclature
1. Find and name the longest continuous carbon chain.
2. Identify and name groups attached to this chain.
3. Number the chain consecutively, starting at the end nearest a substituent
group.
4. Designate the location of each substituent group by an appropriate number
and name.
5. Assemble the name, listing groups in alphabetical order. The prefixes di, tri,
tetra etc., used to designate several groups of the same kind, are not
considered when alphabetizing.
Illustration: Name the following organic compound represented by dot-line structure.
A. 4 2
5 3 1 answer: 2-methylpentane
Cl
B. 6 4 2
5 3 1 answer: 3-chloro-2,4 -dimethylhexane
IUPAC Rules for Alkene and Alkyne Nomenclature
1. Identify the longest carbon chain that contains both carbons of the double or triple
bond.
2. The carbon backbone is numbered from the end that yields the lowest positioning for
the double or triple bond.
3. Substituents are added to the name as prefixes to the longest chain.
4. The position of the double or triple bond is indicated using the position of the carbon in
the bond with the lower backbone number, and the suffix for the compound is
changed to “-ene” for an alkene and “-yne” for an alkyne.
Illustration 1: Name the following organic compound:
Solution:
Assign the lowest number to the double bond
5 3 1
4 2 answer: 2-pentene
Illustration 2: Name the following organic compound
answer: 4-chloro-6-diiodo-7-methyl-2-nonyne
When there are two double or triple bonds in the molecule, find the longest carbon chain including both the triple bonds. Number the longest chain starting at the end closest to the double or triple bond that appears first. The suffix that would be used to name this molecule would be - diene for 2 double bond and –diyne for two triple bonds. For example:
1 3 5 7
name: 4-methyl-1,4-octadiyne
2 4 6 8
Lesson 9.4 Sigma Bond and Pi Bond
Sigma bond is a covalent bond resulting from the formation of a molecular orbital by the end-to-end overlap of atomic orbitals Denoted by the symbol σ.
Pi bond is a covalent bond resulting from the formation of a molecular orbital by side-to-side overlap of atomic orbitals along a plane perpendicular to a line connecting the nuclei of the atoms. Denoted by the symbol π.
Locating σ and π bond
σ - bonds
C C C C C C
π - bonds
Example: How many sigma and pi bonds are there in ethene ?
Hybridization
Hybridization is the fusion of atomic orbitals to form newly hybridized orbitals, which in turn, influences molecular geometry and bonding properties. Hybridization is also an expansion of the valence bond theory. Three types of hydrocarbon compounds will be utilized to illustrate sp3, sp2, and sp hybridization.
Hybridization of Ethane
Ethane is a two carbon molecule with a single-bond between the two carbons To understand the hybridization, consider the orbital diagram of the unhybridized valence electrons of unhybridized carbon.
Carbon has four valence electrons, two in the 2s orbital and two more in three 2p orbitals In ethane molecule, carbon needs to make four single bonds, one to the other carbon atom and three more to the hydrogen atoms. Single bonds can only be made with s-orbitals or hybrid orbitals, and as it stands carbon can not make four bonds. To rectify this the atomic orbitals go through a mixing process called hybridization, where the one 2s and the three 2p orbitals are mixed together to make four equivalent sp3hybrid orbitals . One s orbital and 3 p-orbitals were used in this case, and the result is a total of four sp3 hybrids. The four electrons are then distributed equally among them.
There are two types of orbitals with two types of shapes. A s type orbital is a sphere of electron density around an atom. Hybrids and sorbitals can make sigma type bonds where the electron density is shared directly between the atoms. The other type, p-orbitals, have two lobes above and below the plane of the atom. They are used to make π bonds, which make up double and triple bonds.
sp3 hyrbid orbital
Shown above is the sp3 orbital used by the carbon to make the sigma bond with the adjacent carbon. There are three things to notice:
1) The bulk of the electron density is directly between the two carbon atoms, indicative of a sigma bond.
2) The shape of the hybrid matches what orbitals were used to make it. For this case, sp3 hybrids are 3 parts p orbitals and 1 part s orbital. The end result is an orbital that is mostly p shaped but it a little bit lop-sided.
3) sp3 hybrids take a tetrahedral geometry with an angle between them of 109.5 degrees. Click on one of the ethane pictures above and rotate the 3D image until you can see this geometry.
Hybridization of Ethylene
Unlike methane, ethylene is shaped differently, despite the fact that the carbon in ethylene has the same electron configuration. Evidence shows that the carbon in an ethylene molecule is sp2 hybridized. This means that 1 s orbital is being mixed with 2 p orbitals.
The energy diagram setup this time is different because only 2 p orbitals are being mixed. While creating the energy diagram, be sure not to make mistake as shown at the above right figure. By placing two electrons in the same orbital, Hund's rule is violated. The said rule states that all orbitals among the same energy levels have to be filled with at least one electron before being paired up again. The 2p orbital here is considered low enough energy to be classified within the same energy level as the sp2 orbitals. The figure below portrays the correct way to distribute your electrons.
Notice how that lone electron in the 2p orbital is separate from the electrons in the sp2 orbitals. This is what influence ethylene's shape. The lone electron from each carbon will remain in its respective p orbital and form a pi bond with the other p orbital electron. Thus, ethylene is a planar molecule, with orbitals spaced 120 degree angles apart.
Hybridization of Acetylene ( or ethyne )
Acetylene is an sp molecule. This means that 1 s orbital is being mixed with 1 p orbital. The energy diagram for this setup is shown below::
The lone electrons in the 2p orbitals are not part of the sp orbitals. Instead, each electron is in its respective p orbital, and will bond with its respective p orbital of the other carbon. This in itself will create a sigma bond and two pi bonds, leading to the formation of a linear molecule.
In Lewis structure, acetylene is comprised of 2 triple-bond carbons. The bond angle is 180 degrees, indicative of a linear molecule.
Organic Compounds : Carbon Atom, Hydrocarbons Bonding & Naming, Sigma & Pi bonds , and Hybridization
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