Chemistry 20 Chapters 2 Alkanes ydrocarbons: a large family of organic compounds and they contain only carbon and hydrogen. ydrocarbons are divided into two groups: 1. Saturated hydrocarbon: a hydrocarbon that contains only carbon-carbon single bonds (alkanes, also called Aliphatic hydrocarbons). Saturated in this context means that each carbon in the hydrocarbon has the maximum number of hydrogen atoms bonded to it. 2. Unsaturated hydrocarbon: a hydrocarbon that contains one or more carbon-carbon double bonds, triple bonds, or benzene rings. Alkanes: They are saturated hydrocarbons (they have only carbon-carbon single bonds). The molecular formula of this group is C n 2n+2 (n is the number of carbon atoms). Naming the unbranched alkanes: Chemists have adopted a set of rules established by the International Union of Pure and Applied Chemistry (IUPAC). The IUPAC name for an alkane consists of two parts: 1. A prefix that shows the number of carbon atoms (meth-, eth-, prop-, but-, pent-, hex-, hept-, oct-, non- and dec-). 2. The suffix -ane. C 4 Methane C 2 6 Ethane C 3 8 Propane C 4 10 Butane Note: we can represent the formula of an organic compound by the molecular formula or by the structural formula. Structural formulas can be represented by three ways: Expanded (Complete) structural formula, Condense structural formula, and Line-angle formula: Expanded (complete) structural formula: to represent this model, the carbon atom is shown attached to the hydrogen atoms (we show all connections).
Condensed structural formula: to represent this model, the hydrogen atoms are grouped with their carbon atom. The number of hydrogen atoms is written as subscript. -C 2 -C 2 -C 2 - or -(C 2 ) 3 - Line-angle formula: is a form of the structural formula. A line represents a carbon-carbon bond and a vertex represents a carbon atom. A line ending in space represents a group. -C 2 - Propane -C 2 -C 2 -C 2 - Pentane Substituent groups: they are the branches in organic compounds. A substituent group derived from an alkane by removal of a hydrogen atom is called an alkyl group (R-). Alkyl groups are named by dropping the -ane from the name of the parent alkane and adding the suffix -yl. - Methyl C 2 5 - Ethyl C 3 7 - Propyl Note: some substituents derived from other elements or other spices than alkanes: -F Fluoro -Cl Chloro -O ydroxyl -NO 2 Nitro Naming branched alkanes: 1. Write the alkane name of the longest continuous chain of carbon atoms (parent chain or root chain). 2. Number carbon atoms starting from the end nearest substituent. 3. Give the location and name of each substituent (alphabetical order) as a prefix to the alkane name (main chain). Use a hyphen to connect the number to the name. -C-C 2-1 2 3 4 In this example, the longest chain is Butane. We number carbon atoms starting from the end nearest substituent (left to right). The location of subtituent is 2 and its name is methyl. Therefore, the complete name of this compound is 2-Methylbutane. Cl -C 2 -C-C- 5 4 3 2 1 3-Chloro-2-methylpentane Constitutional isomers: compounds with the same molecular formula but a different connectivity of their atoms (different structural formulas). Butane: C 4 10 -C 2 -C 2 - Methylpropane: C 4 10 -C- Note: Constitutional isomers are different compounds and have different physical and chemical properties.
Cycloalkane: a saturated hydrocarbon that contains carbon atoms bonded to form a ring. A hydrocarbon that contains carbon atoms joined to form a ring is called a cyclic hydrocarbon. = Cyclohexane Cyclopentane Naming of Cycloalkanes: to name a cycloalkane, prefix the name of the corresponding openchain alkane with cyclo-, and name each substituent on the ring. If there is only one substituent on the ring, there is no need to give it a location number. If there are two or more substituents, number the ring beginning with the substituent of lower alphabetical order. Cl O Br Cl NO 2 Chlorocyclohexane 1-Bromo-2-methylhexane 1-Chloro-3-hydroxyl-2- nitrocyclopentane Conformation: any three-dimensional arrangement of atoms in a molecule that results from rotation about a single bond. Note: There is, in fact, free rotation about each carbon-carbon bond in an alkane. As a result, even a molecule as simple as ethane has an infinite number of possibility three-dimensional shapes, or conformations. The majority of molecules in any sample will be in the least crowded conformation; the fewest will be in the most crowded conformation. Least crowded conformation Most crowded conformation Note: The most stable conformation of cyclopentane is the envelope conformation. In it, four carbon atoms are in a plane, and the fifth carbon atom is bent out of the plane. All bond angles in cyclopentane are approximately 109.5. Note: The most stable conformation of cyclohexane is the chair conformation, in which all bond angles are approximately 109.5. Cis-Trans Isomerism in Cycloalkanes: cycloalkanes with substituents on two or more carbons of the ring show a type of isomerism called cis-trans isomerism. These iomers have the same molecular formula and the same connectivity of their atoms, but a different arrangement of their atoms in space (because of restricted rotation around the carbon-carbon single bonds of the ring). The prefix cis indicates that the substituents are on the same side of the ring; the prefix trans indicates that they are on opposite sides of the ring.
3 C or 3 C or trans-1,4-dimethylcyclohexane cis-1,4-dimethylcyclohexane Stereoisomers: isomers that have the same connectivity of their atoms but a different orientation of their atoms in space. Because cis-trans isomers differ in the orientation of their atoms in space, they are stereoisomers. Physical properties of alkanes: 1. They are nonpolar compounds (the electronegativity difference between carbon and hydrogen is 2.5-2.1 = 0.4). 2. The only interactions between their molecules are the very week London dispersion forces. 3. They are insoluble in water (because water is polar) and they are soluble in nonpolar organic compounds. 4. They have the lower density than water (their densities is between 0.7 and 0.8 g/ml). 5. They have the low boiling points and the low melting points. 6. They can be gases (with 1 to 4 carbon atoms), liquids (with 5 to 17 carbon atoms), or solids (with 18 or more carbon atoms). Note: In general, both boiling and melting points of alkanes increase with increasing molecular weight (the number of carbon atoms). Note: In general, both boiling and melting points of alkanes decrease with increasing the number of branches (for alkanes with the same molecular weights). As branching increases, the alkane molecule becomes more compact and its surface area decreases (London dispersion forces act over a smaller surface area). Chemical properties of alkanes: in general, they have a low reactivity (inert). Their most important reactions are combustion (reaction with oxygen) and halogenation (reaction with halogens). Combustion: alkanes react with oxygen (they are oxidized). In this reaction, CO 2, 2 O, and energy (heat) are produced. C 4 + 2O 2 CO 2 + 2 2 O + energy (heat) alogenation: alkanes react with chlorine and bromine if we heat the mixture or if we expose the mixture to light (in the dark at room temperature, nothing happens). C 4 + Cl 2 Cl + Cl Chloromethane If chlromethane is allowed to react with more chlorine: Cl + Cl 2 C 2 Cl 2 + Cl Dichloromethane
C 2 Cl 2 + Cl 2 CCl 3 + Cl Trichloromethane CCl 3 + Cl 2 CCl 4 + Cl Tetrachloromethane Sources of alkanes: the two major sources of alkanes are natural gas and petroleum. Natural gas consists of approximately 90 to 95% methane, 5 to 10% ethane, and a mixture of other relatively low-boiling alkanes (propane, butane and 2-methylpropane). Petroleum is a thick, viscous, liquid mixture of thousands of compounds, most of them hydrocarbons, formed the decomposition of marine plants and animals. The fundamental separation process in refining petroleum is fractional distillation. All crude petroleum that enters a refinery goes to distillation units, where it is heated to temperatures as high as 370 to 425 C and separated into fractions.