Organic Chemistry: Hydrocarbon Compounds Overview

Chapter 22Hydrocarbon Compounds kerosene Diesel fuel gasoline

Organic Chemistry & Hydrocarbons Scientists used to think that only living organisms could synthesize the carbon compounds found in their cells. They called the study of these compounds organic chemistry. However, Friedrich Wohler (1800s) was able to use inorganic substances to synthesize urea. ( a carbon compound found in urine) Today - Organic chemistry includes the chemistry of almost all carbon compounds regardless of their origin.

Hydrocarbons There are many more organic compounds than inorganic compounds The simplest organic compounds contain only carbon and hydrogen and are called hydrocarbons. The two simplest hydrocarbons are methane (CH4) and ethane C2H6. Methane is a major component of natural gas and is formed by the action of bacterial on decaying plants in swamps and other marshy areas. Livestock and termites also emit substantial quantities of methane as a product of digestion.

Methane Carbon has 4 valence electrons and hydrogen has 1, so the carbon atom can form a single covalent bond with four hydrogen atoms. Because carbon has four valence electrons, a carbon atom always forms four covalent bonds.

Ethane Methane is not typical of the vast majority of organic compounds, because there isn’t a bond between carbon atoms in a methane molecule. Ethane has a carbon-carbon bond. Two carbons atoms share a pair of electrons. The remaining six valence electrons form bonding pairs with the electrons from six hydrogen atoms.

Alkanes The ability of carbon to form stable carbon-carbon bonds is one reason that carbon can form so many different compounds. Alkanes are hydrocarbons in which there are only single covalent bonds. In alkanes, all the carbon-carbon bonds are single covalent bonds and all other bonds are carbon hydrogen bonds. The carbon atoms in an alkane can be arranged in a straight change or in a chain that has branches Alkenes have the general formula CnH2n + 2 (C2H6)

Straight-Chain Alkanes Ethane is the simplest of the straight-chain alkanes, which contain any number of carbon atoms, one after the other in a chain. Propane (C3H8) has three carbon atoms bonded in a chain with eight electrons shared with eight hydrogen atoms. Butane (C4H10) has four carbon atoms bonded in a chain with 10 hydrogen atoms. Homologous series – a constant increment of change in molecular structure from one compound in the series to the next. (straight-chain alkanes are an example)

Straight-Chain Alkanes

Naming Alkanes Every alkane has a name that ends with the suffix –ane. For the straight-chain alkanes with 1 to 4 carbon atoms, the official names and the common names are the same. (Methane, ethane, propane, and butane) A mixture of Latin and Greek prefixes are used to name the hydrocarbons having straight chains longer than four carbon atoms. The prefixes are pent- 5, hex- 6, hept- 7) oct- 8, etc. A complete structural formula shows all the atoms and bonds in a molecule. Sometimes, shorthand structural formulas work just as well.

Condensed Structural Formulas In a condensed structural formula, some bonds and/or atoms are left out of the structural formula. (even thought the bonds and atoms do not appear, you must understand that they are there) C4H10 CH3 – CH2 – CH2 – CH3 CH3(CH2)2CH3 C – C – C - C

Branched-Chain Alkanes Because a carbon atoms form four covalent bonds, it can bond not only to one or two other carbon atoms, but also to three or even four other carbons, resulting in branched chains. Branches on a hydrocarbon chain are discusses as if they were substituted for a hydrogen atom on the chain. A atom or group of atoms that can take the place of a hydrogen atom on a parent hydrocarbon molecule is called a substituent. The longest continuous carbon chain of a branched-chain hydrocarbon is called the parent alkane. All other carbon atoms or groups of carbon atoms are regarded as substituents.

Branched-Chain Alkanes Substituent C | C -- C – C Parent alkane (propane) Substituents C C C | | | C --- C -– C -– C –- C --- C Parent alkane (hexane)

Alkyl Groups • A hydrocarbon substituent is called an alkyl group. • An alkyl group can be one carbon or several carbons long. • Alkyl groups are name by removing the –ane ending from the parent hydrocarbon name and adding –yl. • The three smallest alkyl groups are • methyl group (CH3 --) • ethyl group (CH3CH2 --) • propyl group (CH3CH2CH2 --) • An alkyl group consists of an alkane with one hydrogen removed.

Naming Branched-Chain Alkanes When a substituent alkyl group is attached to a straight-chain hydrocarbon, branches are formed. An alkane with one or more alkyl groups is called a branched-chain alkane. The name of a branched-chain alkane is based on the name of the longest continuous carbon chain. Each alkyl substituent is named according to the length of its chain and numbered according to its position on the main parent chain.

Naming Branched-Chain Alkanes 7 6 5 4 3 2 1 CH3 --- CH2 -– CH2 -– CH –- CH --- CH ---CH3 | | | CH2 CH3 CH3 | CH3 4-ethyl-2,3-dimethylheptane

Naming Branched-Chain Alkanes • Find the longest chain of carbon atoms (parent) • Number the carbons in the main chain in sequence. (Start at the end that will give the groups attached to the chain the smallest numbers.) • Add numbers to the names of the substituent groups to identify their positions on the chain. (these numbers become prefixes to the name of the alkyl group) • Use prefixed to indicate the appearance of the same group more than once in the formula. (di-, tri-, tetra-, penta-) • List the names of alkyl substituents in alphabetical order. (ignore the prefixes di-, tri-, etc.) • Use proper punctuation. Commas are used to separate numbers. Hyphens are used to separate numbers and words. Entire name written without any spaces.

Properties of Alkanes The electron pair in a carbon-hydrogen or a carbon-carbon bond is shared almost equally by the nuclei of the atoms form the bond. Molecules of hydrocarbons, such as alkanes, are nonpolar molecules. The attractions between nonpolar molecules are weak van der Waals forces, so alkanes of low molar mass tend to be gases or liquids that boil at a low temperature. The nonpolar hydrocarbon compounds will not form solutions with polar compounds.

End of Section 22.1

Alkenes Organic compounds that contain the maximum number of hydrogen atoms per carbon atom are called saturated compounds. (alkanes) Compounds that contain double or triple carbon-carbon bonds are called unsaturated compounds. Alkenes are hydrocarbons that contain one or more carbon-carbon double covalent bonds. At least one carbon-carbon bond in an alkene is a double covalent bond. Other bonds may be single carbon-carbon and carbon-hydrogen bonds. Alkenes have the general formula CnH2n (C2H4)

Naming Alkenes • Ethene (common name ethylene) is the simplest alkene. • To name an alkene by the IUPAC system: • Find the longest chain in the molecule that contains the double bond. (this will be the parent chain). It has the root name of the alkane with the same number of carbons plus the ending -ene. • The chain is numbered so that the carbon atoms of the double bond have the lowest possible numbers. • Substituents on the chain are name and numbered in the same way they are for the alkanes.

Alkenes Ethene (ethylene) 1- butene Propene

Alkynes Hydrocarbons that contain one or more carbon-carbon triple covalent bonds are called alkynes. Alkynes have the general formula CnH2n-2 (C2H2) The simplest alkyne is the gas ethyne, which has the common name acetylene. Straight chain and branched chain alkanes, alkenes, and alkynes are aliphatic hydrocarbons. The major attractions between aliphatic molecules are weak van der Waals forces. (the introduction of a double or triple bond into a hydrocarbon does not have a dramatic effect on physical properties such as boiling point. )

Structural Isomers Structures of some hydrocarbons differ only in the positions of substituent groups or of multiple bonds in their molecules. Compounds that have the same molecular formula but different molecular structures are called isomers. Isomers have different properties from each other. butane 2-methylpropane

Structural Isomers Structural isomers are compounds that have the same molecular formula, but the atoms are joined together n a different order. Structural isomers differ in physical properties such as boiling point and melting point. They also have different chemical reactivities. In general, the more highly branched the hydrocarbon structure, the lower the boiling point of the isomer compared with less branched isomers.

Stereoisomers Isomers • Stereoisomers are molecules in which the atoms are joined in the same order, but the positions of the atoms in space are different. • There are two types of steroisomers: • Geometric isomers • optical isomers.

Geometric Isomers A double bond between two carbon atoms prevents them from rotating with respect to each other. Because of this lack of rotation, groups on either side of the double bond can have different orientations in space. Geometric isomers have atoms joined in the same order, but differ in the orientation of groups around a double bond.

Geometric Isomers In the trans configuration, the methyl groups are on opposite sides of the double bond. In the cis configuration, the methyl groups are on the same side of the double bond. Trans-2-butene and cis-2-butene have different physical and chemical properties.

Geometric Isomers The groups attached to the carbons of the double bond do not need to be the same. Geometric isomerism is possible whenever each carbon of the double bond has at least one substituent. cis-2-pentene 2-methyl-1-butene Trans-2-pentene

Optical Isomers Whenever a carbon atom has four different atoms or groups attached, optical isomers occur. Asymmetric carbon – a carbon with four different atoms or groups attached. The relationship between the molecules attached to the carbon is similar to the relationship between right and left hands.

Optical Isomers Pairs of molecules that differ only in the way that four different groups are arranged around a central carbon atom are called optical isomers. The molecules cannot be superimposed because they are mirror images of each other.

Hydrocarbon Rings In some hydrocarbon compounds, the carbon chain is in the form of a ring. These are called cyclic hydrocarbons. Cyclopropane Cyclopentane Cyclohexane

Aromatic Hydrocarbons There is a class of organic compounds that are responsible for the aroma of spices such as vanilla, cinnamon, cloves and ginger. These compounds were originally called aromatic compounds because they have distinct pleasant odors. However, not all compounds currently classified as aromatic have an odor. Molecules of aromatic compounds contain a single ring or a group of rings. Benzene (C6H6) is the simplest example of an aromatic compound. Aromatic compound is an organic compound that contains a benzene ring or other ring in which the bonding is like that of benzene.

Aromatic Hydrocarbons Another name for an aromatic compound is an arene. Because of the structure of benzene, the properties of aromatic compounds are quite different from those of aliphatic compounds. Benzene is a six-membered carbon ring with a hydrogen atom attached to each carbon. One electron from each carbon is free to participate in a double bond.

Benzene Structure In a benzene molecule, the bonding electrons between carbon atoms are shared evenly around the ring. Recall that when two or more equally valid structures can be drawn for a molecule, resonance occurs. Benzene and other molecules that exhibit resonance are more stable than similar molecules that do not exhibit resonance. Thus, benzene is not as reactive as six-carbon alkenes.

Substituted Aromatic Compounds Compounds containing substituents attached to a benzene ring are named as derivatives of benzene. When the benzene ring is a substituent on an alkane, the C6H5 group is called a phenyl group. methylbenzene ethylbenzene 3-phenylhexane

Disubstituted Benzenes Some derivatives of benzene have two substituents. These derivatives are called disubstituted benzenes. There are three structural isomers for the liquid aromatic compound dimethylbenzene. Common names for disubstituted benzenes use the terms ortho, meta, and para in place of numbers.

Natural Gas • Much of the world’s energy is supplied by burning fossil fuels. Fossil fuels are carbon-based because they are derived from the decay of organisms. • Typically, natural gas is composed of: • about 80% methane • 10% ethane • 4% propane • 2% butane. • The remaining 4% consists of nitrogen and hydrocarbons of higher molar mass. • also contains a small amount of He) • Methane, the major constituent of natural gas is especially prized for combustion because it burns with a hot, clean flame.

Combustion of Hydrocarbons Propane and butane are separated from the other gases in natural gas by liquefaction and sold in pressurized tank as liquid petroleum gas (LPG) Oxygen is necessary for the efficient combustion of a hydrocarbon. If there is not enough O2 available, the combustion is incomplete. Complete combustion gives a blue flame. Incomplete combustion gives a yellow flame. Carbon monoxide, a toxic gas, is also formed along with carbon dioxide and water during incomplete combustion.

Petroleum The organic compounds found in petroleum, or crude oil, are more complex than those in natural gas. Most of the hydrocarbons in petroleum are straight-chain and branched-chain alkanes. Petroleum also contain small amounts of aromatic compounds and sulfur, oxygen, and nitrogen containing organic compounds. Petroleum is a mixture of hydrocarbons having from one to 40+ carbon atoms. Without further treatment, petroleum is not very useful. The mixture must be separated, or refined, into parts called fractions.

Petroleum Fractionation The refining process starts with the distillation of petroleum (crude oil) into fractions according to boiling point.

Hydrocarbon Cracking The amounts of products obtained by fractional distillation are not in proportion to the demand of the market. Gasoline is the most commonly used product, so other processes are used to make the supply meet the demand. Cracking is a controlled process by which hydrocarbons are broken down or rearranged into smaller, more useful molecules. (with the aid of a catalyst and with heat) Example: fractions containing compounds of higher molar mass are “cracked” to produce the more useful short-chain components of gasoline and kerosene.

Coal Formation - Peat The first stage in coal formation is an intermediate material known as peat. Peat is a soft, brown, spongy, fibrous material with a very high water content. When first dug out peat has a very high water content. After it is allowed to dry, it produces a low cost but smoky fuel.

Coal Formation - Lignite If peat is left in the ground, it continues to change. After a long period of time, peat loses most of its fibrous texture and becomes lignite, or brown coal. Lignite is much harder than peat and has a higher carbon content (about 50%). The water content is still high.

Coal Formation - Bitumious Continued pressure and heat slowly change lignite into bitumious, or soft coal. Bitumious coal has a lower water content and higher carbon content (70 – 80%) than lignite.

Organic Chemistry: Hydrocarbon Compounds Overview