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Carbon chemistry - Basics

Hydrocarbons, organic compounds, inorganic compounds, organic chemistry

Elements come together and make compounds. Carbon and hydrogen come together to form hydrocarbons.

Many a times we hear about organic compounds. What are they? It is quite simple to know that the compounds which are obtained from plants and animals or simply organisms are called organic compounds or carbon compounds. The chemistry of organic compounds is called as organic chemistry.

On the other hand there are compounds which are obtained from minerals. These are called inorganic compounds.

Organic compounds from inorganic compounds

Are organic compounds formed exclusively from organisms (plants and animals)? The answer is no.

Organic compounds can be made artificially. They can be synthesized in laboratory. Hence they can be manmade.

The best example is that of urea CO(NH2)2 which is an organic compound.

German chemist Friedrich Wohler synthesized urea from an inorganic compound ammonium cyanate NH4(NCO).

Hydrocarbons

Hydrocarbons or organic compounds contain hydrogen along with carbon. These are called as parent compounds or fundamental organic compounds. Sometimes organic compounds also contain oxygen, halogens, nitrogen and sulphur.

Methane: Its formation, covalent bonds and structure

Methane (CH4) is the simplest hydrocarbon or alkane. Popularly it is called marsh gas since it is produced from the decaying matter in marshes. It has one C and four H atoms.

The electronic conﬁguration of carbon is 2, 4 and of hydrogen is 1. Carbon has four electrons in its outermost or second orbit (shell). To complete the octet and attain stability carbon bonds with four hydrogen atoms. Hydrogen too requires one electron in its first orbit to attain stability. This is how tetravalent carbon and monovalent hydrogen come together and satisfy their valencies.

The electrons are shared by both carbon and hydrogen. The bond formed by sharing of electrons is a covalent bond. A single covalent bond is formed by sharing of two electrons, one electron each of carbon and hydrogen. One carbon atom forms four bonds with four hydrogen atoms. This leads to the formation of four C-H single covalent bonds.

The structure of methane is

In the structure carbon atom is centrally placed.

If the electrons of carbon are shown by "x" and the electrons of hydrogen by ". " then the structure of methane looks like

This structure is known as “electron-dot and cross” structure.

Covalent bonds

Covalent bonds are formed by the sharing of electrons. Covalent bonds are weak bonds.

Organic compounds with covalent bonds exhibit the following properties:

(1) They have low melting and boiling points,

(2) They are generally insoluble in water but are soluble in other organic solvents,

(3) They are poor conductors of heat and electricity.

Covalent bond in oxygen

The atomic number of oxygen is 8. Its electronic configuration is 2, 6. Six electrons are present in its outermost shell. It requires two more electrons to complete its octet. Oxygen is divalent. When two oxygen atoms come close they try to attain stability by sharing their electrons.

Each oxygen atom shares its valence electron with the valence electron of another oxygen atom. This gives two shared pairs of electrons which results in the formation of O2 molecule.

If two electron pairs are shared between two atoms, then a double covalent bond (=) is formed. The structure of oxygen molecule is O::O or O = O or O2.

Saturated and unsaturated hydrocarbons

Saturation means to become full.

Saturated hydrocarbons: Methane has four single covalent bonds. When there is a single bond between carbon and hydrogen the compound or hydrocarbon is called a saturated hydrocarbon. In case of ethane (C2H6) and higher alkanes, carbon atoms are linked to each other by single covalent bonds.

Unsaturated hydrocarbons: Hydrocarbons in which carbon atoms are linked to each other by double or triple bonds are known as unsaturated hydrocarbons e.g. alkenes and alkynes.

Types of hydrocarbons and there bonding

Saturated and unsaturated hydrocarbons can form straight chains or closed chain structures. These chains can have branches and cross links and are known as closed chain or ring compounds.

Catenation

Catenation is the linkage of atoms of the same element into longer chains. Carbon atom forms bonds with other carbon atoms. The carbon chains can be straight or branched forming large molecules.

Allotropes of Carbon

Allotropy: The phenomenon of existence of a substance in various physical forms but same chemical form e.g. diamond and graphite.

Diamond and graphite – differences, similarities, uses

Parent hydrocarbons or alkanes

The general formula for alkanes is CnH2n+2 where n is the number of carbon atoms e.g.

methane CH4

ethane C2H6

propane C3H8

Isomers and isomerism

Compounds with identical molecular formula but different structure are called isomers. The property is known as isomerism.

Straight chain and branched chain

Pentane C5H12 has three possible carbon skeletons.

The carbon atoms are linked together in the form of open chain. These compounds also contain branched chain.

Structure of Benzene C6H6 Benzene ring is made up of six carbon atoms, in which each carbon atom is joined by a single bond on one side and double bond on other side (alternate single bond and double bonds)

Functional groups in organic compounds

All organic compounds are derived from hydrocarbons. These are called derivatives. The derivatives are formed by replacing one or more hydrogen atoms in a molecule of hydrocarbon by some other atom or group of atoms. After replacement, a new set of compounds is formed which has functions (properties) different from the parent hydrocarbon.

Functional group is an atom or group of atoms present in the molecule which determines the characteristic property of organic compounds.

In methane CH4 if one hydrogen is replaced by an -OH group, then a compound known as methyl alcohol is formed. The -OH group is known as the alcoholic functional group.

Homologous series

It is a group of compounds with similar structural features that can be represented by a general formula whose members differ by one -CH2- unit

A group of organic compounds containing same functional group, which can be represented by the same general formula and which more or less show similar trends in their properties

e.g. methane CH4, ethane C2H6 or CH4CH2, propane C3H8 or C2H6CH2

Important characteristics of homologous series

(1) The general formula of all compounds in the series is the same CnH2n+2

(2) They have the same functional group

(3) Physical properties like melting point, boiling point, density, generally show a gradual change with increase of molecular formula in the series. On the other hand, chemical properties of the member show close resemblance because of the presence of the same functional group in them

(4) Consecutive members of the series differ from one another by -CH2- group which is known as the methylene group and their molecular weight differs by 14 units (carbon atomic weight 12, hydrogen atomic weight 1 X 2 atoms = 2, CH2 = 14)

Example: If molecular formulae are arranged in order of increasing number of carbon atoms in their molecule, the following series is obtained.

Alkanes: The Alkane family is a homologous series and characterized by the general formula CnH2n+2

Alcohols CnH2n+1OH

Nomenclature of organic compounds

According to IUPAC (lnternational Union of Pure and Applied Chemists) all organic compounds are considered as derivatives of saturated hydrocarbons or Alkanes.

Terminology

Root: It indicates the nature and the number of carbon atoms in the basic carbon skeleton.

Suffix (added after the word): It denotes the type of bonds or functional group present in the carbon chain. A Sufﬁx is added to a root word to indicate the saturation or unsaturation in the carbon chain.

Prefix (added before the word): It indicates the presence of other functional groups and their position.

E.g. C2H5OH

The longest chain is of two carbon atoms
The name of the parent alkane is ethane
One hydrogen atom is substituted by the -OH group
Since the functional group is alcohol, remove the ‘e’ from the word ethane and substitute it with ‘ol’ ('ol' stands for alcohol)
The carbon atom to which the -OH group is attached is numbered as C1 and the other carbon atoms are numbered accordingly
The compound C2H5OH is named as ethan-1-ol indicating that the functional group-OH is attached to the carbon atom at the end of the chain

E.g. 2-bromopropane 1-bromopropane

The longest chain is of three carbon atoms.
The name of the parent alkane is propane. As the carbon atoms are bonded in a straight line, it is a straight chain compound.
One hydrogen atom in the chain is substituted by the -Br group. The functional group is halide (in this case bromo)
The carbon atom nearest to the substituted group is numbered as C1.

The compound on the left is called 2-bromopropane indicating that the -Br group is attached to the second carbon atom. The compound on the right is called 1-bromopropane indicating that the -Br group is attached to the first carbon atom.

E.g. Unsaturated compound containing a double bond

CH3CH2CH=CH2 Butene-1 or But-1-ene

4 3 2 1

The longest chain of carbon atoms is four and it is in a straight chain.
The parent alkane is butane but for unsaturated hydrocarbons, the sufﬁx-ene- is added. So the above chain is named as butene.
In the structure, the numbering of carbon atoms starts from the carbon atom nearest to the double bond. In the above case, the carbon atom on the extreme right is numbered 1.
The position of the double bond in the chain is indicated by preﬁxing the lower number of the carbon atoms between the double bonds. In the above case, since the double bond is between C1 and C2, the compound is known as but-1-ene or butene-1

E.g. Unsaturated compound containing a triple bond

H-C≡C-H ethyne/ acetylene

In this case the sufﬁx is changed from -ene- to -yne-

Chemical properties of carbon compounds

Combustion

All hydrocarbons burn in air or oxygen to form CO2 and H20. The reactions are exothermic with the evolution of a large amount of heat.

For example

(1) CH4(g) + 2O2(g) → CO2(g) + 2H2O (g) + Heat and light

(2) 2C4H10(g) + 13O2(g) → 8CO2(g) + 10H2O(l) + 2658 KJ Heat exothermic combustion

If very limited air is supplied, then methane gives carbon black.

CH4 + O2 → C + 2H20

Limited Air Carbon black

Saturated hydrocarbons give a clean ﬂame (oxidizing flame) but when unsaturated carbon compounds burn, they give a yellow ﬂame (reducing ﬂame) with lots of black carbon.

Addition Reaction

(a) The reaction in which two molecules react to form a single product is known as addition reaction. This type of reaction occurs only in unsaturated compounds where there are double or triple bonds. Reactant adds to the carbon atoms of C=C and C≡C.

(b) Addition of Hydrogen: Addition of hydrogen molecule to ethene gives corresponding ethane i.e. saturated product (unsaturated compound gets converted into saturated compound).

Substitution Reaction

Reactions where substitution of one (or more atoms) in a molecule for another atom takes place are called substitution reactions.

CH4 + Cl2 UV rays → CH3Cl + HCl (H substituted by Cl)

CH3CH2I + KOH → CH3CH20H + KI (I substituted by OH)

Important Carbon Compounds

Ethanol C2H5OH Ethanol is called ethyl alcohol or spirit. It has a linear structure CH3CH2OH.

Properties

Ethanol is a colourless liquid and has a pleasant odour. Boiling point is 78 °C and freezing point is -114 °C. It is combustible and burns with blue ﬂame.

Some reactions with ethyl alcohol:

(a) Action with sodium metal: When sodium comes in contact with ethyl alcohol it gives hydrogen gas.

2C2H5OH + 2Na → 2C2H5ONa + H2

(ethyl alcohol + sodium → sodium ethoxide + hydrogen)

(b) Action with PCI3: When ethyl alcohol reacts with PCI3 it forms ethyl chloride

3C2H5OH + PCI3 → 3C2H5Cl + H3PO3

(ethyl alcohol + phosphorous trichloride → ethyl chloride + phosphorous acid)

Ethanoic Acid (acetic acid) C2H4O2 It has a linear structure CH3COOH. It is a weak acid, colourless, corrosive liquid and has a pungent smell at ordinary temperature. Below 290 K it solidiﬁes to an ice like mass called glacial acetic acid.

(a) Reaction with halogens

When acetic acid reacts with chlorine monochloroacetic acid is formed.

CH3COOH + Cl2 → CH2CICOOH + HCI Monochloroacetic acid

CH2CICOOH + Cl2 → CHCI2COOH + HCI Dichloroacetic acid

CHCI2COOH + Cl2 → CCl3COOH + HCI Trichloroacetic acid

(b) Reaction with metals

When acetic acid reacts with Na or Zn it gives sodium acetate with liberation of hydrogen gas. (acetate CH3COO)

2CH3COOH + 2Na → 2CH3COONa + H2

When acetic acid reacts with ethyl alcohol in presence of anhydrous ZnCl2, ethyl acetate is formed.

CH3COOH + C2H5OH anhydrous ZnCl2 → CH3COOC2H5 + H2O

Soaps and Detergents

Soaps are cleansing agents which are capable of reacting with water to dislodge the unwanted particles from cloth or skin. The molecules of soap are sodium or potassium salts of long chain carboxylic acids (R-COOH; carboxyl group COOH).

A soap molecule has a tadpole shaped structure. At one end (long non-polar end/ tail) of soap molecule is a hydrocarbon chain that is insoluble in water (hydrophobic = dislike for water) but soluble in oil. At the other end (short polar end/ head) of soap molecule there is a carboxylate (salt or ester of a carboxylic acid) ion which is hydrophilic (love for water) i.e. water soluble but insoluble in oil.

When soap is mixed with water, the solution becomes concentrated and causes foaming. The hydrophobic long non-polar end of soap (tail) gravitates towards and traps the dirt (oil). This is called as micelle. The short polar end (head) with the carboxylate ion turns the water away from the dirt. The soap molecule thus helps in removing the dirt.