Hydrocarbons Chapter 8:
What is carbon chemistry? • Compounds containing Carbon make up 90% of all chemicals and form the basis of living things • Organic chemistry is the study of Carbon compounds
How does Carbon form so many compounds? The electronic configuration of carbon is 1s22s22p2. It can form a wide variety of compounds because: • each carbon atom has four valence electrons, all available for bonding with other atoms • a carbon atom can form strong covalent bonds with other carbon atoms • bonds between carbon atoms can be single or multiple.
Hydrocarbons • Compounds made up of Hydrogen and Carbon are known as Hydrocarbons • Hydrocarbons can be classified into several series or families • The first series is known as the “alkanes”
Alkanes • Are a family that consist of carbon and hydrogen only • They contain only single bonds • CnH2n+2 • Compounds that differ only by –CH2- belong to the same homologous series • Compounds of the same homologous series share the similar chemical properties
Representing Alkanes We use structural formulas to represent hydrocarbons You will notice that each carbon atom: • forms a single covalent bond to four other atoms • each hydrogen atom forms a single covalent bond to one carbon atom • the four atoms bonded to each carbon atom are arranged in a tetrahedral manner around the carbon.
Structural Isomers When we come to draw a structural formula for C4H10, however, there are two possible arrangements that satisfy the bonding requirements of each of the four carbon atoms and ten hydrogen atoms
Structural Isomers These two compounds are structural isomers. That means that they have the same molecular formula (C4H10) but a different arrangement of their atoms. Structural isomers have similar chemical properties but differ in some physical properties such as melting and boiling temp. As molecules become larger, the number of possible arrangements of atoms increases rapidly.
Saturated Hydrocarbons The alkanes are known as saturated hydrocarbons. Because there are only single bonds between carbon atoms, they are ‘saturated’ with hydrogen atoms. Carbon and hydrogen can also form families of compounds in which there are double or triple bonds between carbon atoms. These compounds are unsaturated as they do not contain the maximum number of H atoms.
Naming Alkanes Alkanes use the prefix relevant to the number of Carbons, and ends in “ane”. Four things you may be asked for: • Name the compound • Write the molecular formula • Draw the Structural formula • Write the condensed structural formula (semi-structural)
Alkenes The alkenes form a new homologous series. Their members differ by -CH2- and contain a single double bond between two carbon atoms and share similar chemical properties. CnH2n Alkenes are unsaturated as they contain less than the maximum amount of Hydrogens possible. They are named using the same prefix and end in “ene”. Isomers exist in Alkenes as well. Isomers can be branched, straight chain or even a different position of the double bond.
Your Turn: • Complete the Handout • Name the alkanes • Draw the Structural formula • Write the semi-structural (condensed) formula • Write the molecular formula • Chapter 8 Review Questions • Q 2, 3, 4, 5, 6 • Complete the second handout of challenge questions Don’t forget – SACT due next Wednesday!! Check the blog and keep up to date!!
Naming Organic Compounds IUPAC (International Union of Pure and Applied Chemisty) created a common set of naming rules • Rules: • Determine the longest chain of carbon atoms • Determine which end is nearest to a branch, double bond or triple bond • Number the carbon atoms from the end chosen • Name any branches first with the ending –yl, then the longest chain, then any single or double bond • When two or more branches occur on the same carbon atom, the number of the carbon atom is indicated for each branch • When two or more identical branches occur on different carbon atoms, the prefixes di, tri and tetra are used • Avoid the common errors: • Not identifying the longest chain • Not listing the side branches in alphabetical answer • Omitting the prefixes di, tri and tetra
Naming Organic Compounds There are a set of rules used to name carbon compounds to distinguish between different isomers In the systematic naming of straight-chain hydrocarbons, the first part of the name refers to the number of carbon atoms in one molecule The name of the hydrocarbon ends in: • ane if all carbon-to-carbon bonds are single bonds • ene if one of the carbon-to-carbon bonds is a double bond • eyne if one of the carbon-to-carbon bonds is a triple bond
Example - Naming straight chain alkenes To name straight-chain alkenes, first number the carbon atoms in the chain, starting at the end that will give the first carbon atom involved in the double bond the smallest number possible. The numbering of the chain starts at this end and the isomer is named according to the first carbon atom involved in the double bond. But-2-ene But-1-ene
Branched Hydrocarbons An alkyl group most often forms a branch in a branched-chain hydrocarbon. An alkyl group is an alkane molecule less one hydrogen atom and is named after the alkane from which it is derived. For example, –CH3 is a methyl group, • Systematic naming requires us to: • identify the longest continuous chain of carbon atoms in the molecule and the series the molecule belongs to (alkane/alkene/alkyne) • identify the side group that forms the branch in the chain • number the carbon atoms from one of the ends of the longest carbon chain so that the side group is attached to the carbon atom with the smallest number possible.
Your Turn – practice makes perfect! Review Questions: 1. Chapter 8: • Q7 • Q8 (draw as well as name each of these) 2. Write the structural formula for: a) hex-2-ene b) 2-methylpropene c) 2-pentyne d) 4-methylpent-2-ene e) 2,4-dimethylhexane f) 3,3-dimethylpentane g) 3-ethyl-2,4-dimethyloctane h)3-ethylheptane i) 4-ethyl-3-methylhex-2-ene j) 3-ethyl-4,5-dipropyloctane
Functional Groups The majority of carbon compounds contain other elements in addition to hydrogen. Taking an alkane and replacing a hydrogen atom with another atom or group of atoms changes the chemical properties of the compound. An atom or group of atoms that influence the properties of the compounds forming a homologous series is known as a functional group
Hydroxyl group - OH Alcohols have the functional group OH. A functional group is a group of atoms that gives the molecule unique chemical properties. Molecules with the hydroxyl group (OH) are very soluble in water, depending on their molecular size. A “H” atom is replaced by an OH group The last part of the name is replaced with an “ol” MethanolButanol
Carboxyl group - COOH Carboxylic acids have the functional group COOH. Molecules with the carboxyl functional group (COOH) are known as organic acids. They dissolve in water to form acidic solutions. The COOH group is made up of: 2 H atoms are replaced by “COOH” group The last part of the name becomes “-anoic acid” Propanoic Acid Ethanoic Acid
Halogens - (Cl, Br, I, F) Cl – chloroalkanes Br - Bromoalkanes F – Fluoroalkanes I - Idoalkanes A “H” atom is replaced by a Halogen The first part of the halogen is said in front of the alkane Chloroethane 2-Bromopropane
Physical Properties As the number of carbon atoms increases, members of a homologous series show a gradual change in their physical properties. Compounds with smaller molecules are more volatile than larger molecules Less energy is needed to break bonds of smaller molecules Smaller molecules have a lower viscosity The boiling temperature increases as molecules get larger This is true for Alkanes and Alkenes. If an alkane and alkene have the same number of carbons then they will have a similar boiling temp, although an alkenes will be slightly lower.
Physical Properties – WHY? The bonding within hydrocarbons is covalent (strong), however, hydrocarbons are non-polar. The forces between molecules are dispersion forces which increase in strength as the molecule gets bigger. More energy is required to overcome the dispersion forces between larger hydrocarbon molecules, so a higher temperature is required to boil Volatility (tendency to evaporate): The stronger the dispersion forces between molecules in the liquid state, the lower the tendency for the liquid to evaporate and so the lower the volatility. The higher viscosity of the longer-chain hydrocarbons is due to the tendency of longer molecules to become ‘tangled’ together
Chemical Properties of Alkanes The most significant reaction of alkanes for our society is combustion. Alkanes burn in oxygen, releasing large quantities of energy. If the supply of oxygen is sufficient, the products of the reaction are carbon dioxide and water. We use the energy released by the combustion of alkanes as a source of heat, to produce electricity for domestic and industrial use and in transportation.
Combustion Reactions Don’t forget to add in the states (g, aq, l)