The importance oforganic chemistry • Many areas rely on organic chemistry, including: • Biology • Petroleum • Polymers • Genetic Engineering • Agriculture • Pharmacology • Consumer Products
Importance of carbon • Basis for all life. • Form stable covalent bonds to other carbon atoms - catenation. • Can form single, double and triple bonds. • Long carbon chains can be produced. • Will bond with many other elements. • A HUGE number of compounds is possible.
Hydrides of carbon • Catenation • The formation of chains of atoms of the same element. • This key feature of carbon permits a vast number of compounds to exist. • One simple class of compound is the alkane which has only C, H and single bonds. • methane ethane propane butane
Formulas and models • Organic molecules can have very complex structures. • A number of formats are used to represent organic compounds. • Each has its own advantages but the goal is the same, to accurately describe the structure of a compound. • Lets look at some different representations.
Formula • Condensed structural formula • Shorthand way of writing formula. • Lists all atoms in order and tells how they are bound together. • Example. Propane • CH3CH2CH3 • This is a convenient format for describing a molecule using text.
Structural (Constitutional) isomers • Compounds with the same number and type of atoms but with different arrangements. • Molecular Formula C5H12 • Condensed structural formulas. • CH3CH2CH2CH2CH3 pentane • CH3CH(CH3)CH2CH3 2-methylbutane • (CH3)4C 2,2-dimethylpropane • All are structural isomers of C5H12.
OH N H H H H H H H H H 2 2 2 2 2 2 2 2 C C C C C C C C H 2 OH N H H C CH H C H C H C H C H C C 2 2 2 2 2 2 2 C C C C C C C C CH 3 H H H H H H H H 2 2 2 2 2 2 2 2 Line formula • Similar to structural formula. • Each line represents a bond. • Carbons are assumed to be present at the end of each line segment. • Hydrogen is not shown when bound to carbon.
Models • Three dimensional representations Ball and Stick Space Filling Both are models of propane.
Alkanes • Simplest members of the hydrocarbon family. • contain only hydrogen and carbon • only have single bonds • All members have the general formula of • CnH2n+2 Twice as many hydrogen as carbon + 2
Alkanes • First four members of the alkanes • Name # of C Condensed formula • Methane 1 CH4 • Ethane 2 CH3CH3 • Propane 3 CH3CH2CH3 • Butane 4 CH3CH2CH2CH3 • Called a homologous series • “Members differ by number of CH2 groups”
Alkanes • Physical Properties • Nonpolar molecules • Not soluble in water • Low density • Low melting point • Low boiling point These go up as the number of carbons increases.
12.7 Properties of Alkanes • Odorless or mild odor; colorless; tasteless; nontoxic • Flammable; otherwise not very reactive • The first four alkanes are gases at room temperature and pressure, alkanes with 5–15 carbon atoms are liquids; those with 16 or more carbon atoms are generally low-melting, waxy solids.
The boiling and melting points for the straight-chain alkanes increase with molecular size.
Alkanes • Name bp, oC mp, oC Density at 20 oC • Methane -161.7 -182.6 0.000 667 • Ethane - 88.6 -182.8 0.001 25 • Propane - 42.2 -187.1 0.001 83 • Butane -0.5 -135.0 0.002 42 • Pentane 36.1 -129.7 0.626 • Hexane 68.7 - 94.0 0.659 • Heptane 98.4 - 90.5 0.684 • Octane 125.6 - 56.8 0.703 • Nonane 150.7 -53.7 0.718 • Decane 174.0 -29.7 0.730
Sources of alkanes Alkanes can be obtained by refining or hydrogenation of: petroleum shale oil coal Low molecular mass alkanes can be obtained directly from natural gas.
Reactions of alkanes • Combustion • CH4(g) + 2O2(g) CO2(g) + 2H2O(g) • Many alkanes are used this way - as fuels • Methane - natural gas • Propane - used in gas grills • Butane - lighters • Gasoline - mixture of many hydrocarbons, not all alkanes
Reactions of alkanes • Halogenation • A reaction where a halogen replaces one or more hydrogens. • CH4(g) + Cl2(g) CH3Cl(g) + HCl(g) • Used to prepare many solvents • dichloromethane - paint stripper • chloroform - once used as anesthesia • 1,2-dichloroethane - dry cleaning fluid heat or light
Organic nomenclature • Organic molecules can be very complex. • Naming system must be able to tell • Number of carbons in the longest chain • The location of any branches • Which functional groups are present and where they are located. • TheIUPAC Nomenclature System provides a uniform set of rules that we can follow.
I see much memorization in your future! Base names • Prefix Carbons • Meth- 1 • Eth- 2 • Prop- 3 • But- 4 • Pent- 5 • Hex- 6 • Hept- 7 • Oct- 8 • Non- 9 • Dec- 10
Naming AlkanesIUPAC Rules • Acylcic, saturated hydrocarbons end in “-ANE” • Named by the number of carbons in the “chain” • The Root is the longest carbon chain • Substituents or alkyl groups named by the number of carbons • Substituents are identified by the number of the carbon and how many of that type of group. (mono = 1, di = 2 , tri = 3, tetra = 4 etc.) • Two or more different substituents are listed alphabetically • ethyl before methyl • Names are one word. Numbers are separated by commas and names are separated by hyphens. There are no spaces between the substituent and the root names
12.6 Naming Alkanes • The system of naming now used is one devised by the International Union of Pure and Applied Chemistry, IUPAC. • In the IUPAC system for organic compounds, a chemical name has three parts: prefix, parent, and suffix.
Drawing Organic Structures Condensed structure: A shorthand way of drawing structures in which C-C and C-H bonds are understood rather than shown.
Branched-chain alkanes • STEP 1: Name the main chain. Find the longest continuous chain of carbons, and name the chain according to the number of carbon atoms it contains. • The longest chain may not be immediately obvious because it is not always written on one line; you may have to “turn corners” to find it.
STEP 2: Number the carbon atoms in the main chain. Begin at the end nearer the first branch point: • STEP 3: Identify the branching substituents, and number each according to its point of attachment to the main chain:
If there are two substituents on the same carbon, assign the same number to both. There must always be as many numbers in the name as there are substituents. • STEP 4:Write the name as a single word, using hyphens to separate the numbers from the different prefixes and commas to separate numbers if necessary. If two or more different substituent groups are present, cite them in alphabetical order.
If two or more identical substituents are present, use one of the prefixes di-, tri-, tetra-, and so forth, but do not use these prefixes for alphabetizing purposes.
Examples C-C-C-C-C-C | | C-C C • C-C-C-C-C-C-C • | | • C-C C • C-C-C-C C-C-C • | | • C-C-C-C-C-C-C • | • C
Examples C-C-C-C-C-C | | C-CC • C-C-C-C-C-C-C • | | • C-C C 3,5-dimethylheptane • C-C-C-C C-C-C • | | • C-C-C-C-C-C-C • | • C
Examples C-C-C-C-C-C | | C-C C • C-C-C-C-C-C-C • | | • C-C C 3,5-dimethylheptane 3-ethyl-5-methylheptane • C-C-C-C C-C-C • | | • C-C-C-C-C-C-C • | • C
Examples C-C-C-C-C-C || C-C C • C-C-C-C-C-C-C • | | • C-C C 3,5-dimethyl heptane 3-ethyl-5-methylheptane • C-C-C-C C-C-C • | | • C-C-C-C-C-C-C • | • C 2,3,3,7,8-pentamethyldecane
Another example Name the following. (CH3)2CHCH2CH2CH(CH3)2 This is a condensed structural formula. First convert it to a carbon skeleton, leaving out the hydrogens.
Another example (CH3)2CHCH2CH2CH(CH3)2 C C | | C - C - C - C - C - C Now name it!
Another example C C | | C - C - C - C - C - C 1. Longest chain is 6 - hexane 2. Two methyl groups - dimethyl 3. Use 2,5-dimethylhexane
The situation is more complex for larger alkanes. • There are two different three carbon alkyl groups, there are four different four carbon alkyl groups.
Substituents to a Carbon Chain • Alkyl Chains • Named by number of carbons • Straight chains • Branched chains • iPr • sBu • iBu • tBu • Halides • Branch name • Halohydrocarbons
Naming alkyl halides • 1. Follow the same system as with alkanes. • 2. Give the name and carbon number for the halide just like a side branch. • C - C - F C - C - C C-C-C-C-C • | | • Cl C-Br 1-fluoroethane 2-chloropropane 1-bromo-2-ethylbutane
Practice • 2,3 dimethylpentane • 3-ethyl-4-methlylheptane • 4-propyl-2,2,3,3-tetramethyloctane • 4-isopropyloctane
Drawing and Naming Cycloalkanes • Ring structures are possible and very important in organic chemistry. • A more streamlined way of drawing structures is often used in which cycloalkanes are represented simply by polygons.
In line structures, a C is located at every intersection, and the number of H atoms necessary to give each C four covalent bonds is understood. Methylcyclohexane, for example, looks like this in a line structure:
STEP 1: Use the cycloalkane name as the parent. • named as alkyl-substituted cycloalkanes • rather than as cycloalkyl-substituted alkanes. • If there is only one substituent on the ring • it is not necessary to assign a number because all ring positions are identical.
STEP 2: Identify and number the substituents. • Start numbering at the group that has alphabetical priority, • Proceed around the ring in the direction that gives the second substituent the lower possible number.
1,2,4,4,5-pentachlorocylooctane.Draw • Start by drawing an octagon. • Number the carbons and draw in substituents
Cyclic Alkane Properties • Cyclic compounds have ring strain. • They are more eclipsing than linear molecules and can’t rotate to relieve strain. • Also to convert the tetrahedral bond angles to the angles necessary for a ring causes bond-angel strain. • Cyclopropane is planer and unstable. • Cyclobutane is not planer but “puckered” • Cyclopentane has the “envelope” and “half-chair” shapes rather than being planer. • Cyclohexane is not planar as well, has “chair” and “boat” formations. • All to relieve stress and lower potential energy.
Conformations of Cylcohexane Axial groups are close in space, so the smallest substituent will be in the axial postion Chair Conformation
The Chair Conformation Can Flip All the axial positions become equatorial after a flip
Methyl Cylohexane ↔ Flip Which confirmation is favored? Can you draw a Newman projection of carbon 1 and 2 for each conformation?
Multiple bonds • Another key feature of carbon is its ability to form double and triple bonds. • This can be between two carbons • alkenes (C=C) and alkynes (C C) • It can also be between carbon and another element. • C=O • C=N- • C N