Chapter 11 - Introduction to Organic Chemistry: Alkanes (Chapter 11 and 12 from 1 st Edition)

Transcription

1 hapter 11 - Introduction to rganic hemistry: Alkanes (hapter 11 and 12 from 1 st Edition) h11 p1 rganic chemistry is the chemistry of the compounds of carbon. The chemistry of carbon compounds is extremely rich and complex and examples can be found all around us. Gasoline, plastics, medicine, and textiles are but a few examples. Perhaps the greatest example of the richness and complexity of organic compounds comes in the form of DNA. Because organic compounds are found in everyday life, it is important to understand their structure and behavior. What distinguishes one compound from another? Why are some compounds toxic to living organisms? an we predict the reactivity of organic compounds? It is these types of questions that are found at the heart of rganic hemistry. Structural Formulae of rganic ompounds. Because of the rich diversity of organic compounds, it is important to show the proper connectivity of atoms when drawing organic structures. For example, organic chemists draw ethanol as:

2 h11 p2 This structure is an expanded structure of ethanol and it clearly shows the connectivity of all the atoms. The problem with expanded structures is that with larger molecules they can be cumbersome to draw and difficult to read. Because of this, chemists have developed different methods to represent molecules. In a condensed structure, hydrogen atoms are grouped with the atom that they are attached to. Therefore, the condensed structure of ethanol would look like: 3 2 In a line-bond structure, hydrogen atoms attached to carbon are omitted and carbon-carbon bonds are represented as lines. In a line bond structure, carbon atoms are located at the ends of the lines and the vertices where the lines meet. The line bond structure for ethanol is: Important: If you draw a c, you must draw all the atoms attached to it. or or but not or

3 e.g. omplete the following table. h11 p3 Expanded ondensed Line-Bond Formula rganic ompounds. Since organic compounds primarily consist of carbon, hydrogen, oxygen, nitrogen, sulfur, and the halogens, it is important that you understand the types and number of bonds that these elements form. Recall in hapter 4 that carbon has four valence electrons which it can share to form four covalent bonds. ydrogen and the halogens only form one covalent bond. In either case they acquire a noble gas configuration (helium through to xenon). Nitrogen forms three covalent bonds and oxygen and sulfur typically form two covalent bonds. Shown below are some typical examples. It is important that you

4 h11 p4 become familiar with the number of covalent bonds most often formed by these elements. l l N N S l l S S Functional Groups. The amount of organic compounds found in nature number in the millions and more are synthesized each day. Fortunately, this vast number of organic compounds can be organized via characteristic structural features called functional groups. Functional groups are certain groups of atoms that undergo similar reactions. The functional groups also allow us to systematically name each organic molecule according to their family. Functional groups can be broken down into three main categories. 1. Functional groups with carbon in multiple bonds. alkenes (contains =) alkynes (contains ) aromatic (has alternating - and = bonds in a six atom ring)

5 h11 p5 2. Functional groups with heteroatoms (atoms other than carbon or hydrogen) in single bonds. haloalkane (contains -X, where X = F, l, Br, or I) ether (contains --) amine (contains -N) alcohol (contains --) phenol (contains -- where is part of an aromatic ring) thiol (contains -S-) 3. Functional groups with heteroatoms in multiple bonds. ketone (contains = with two attached) aldehyde (contains = with attached) carboxylic acid (contains = with attached) ester (contains = with - attached) amide ( contains = with N attached) Because = appears in so many functional groups it has a special name. It is called a carbonyl group. A carbonyl group itself is not a functional group but is part of many functional groups. Although Timberlake considers alkanes as a functional group, we will not. onsider alkanes as molecules without functional groups. It is important that you become familiar with these functional groups. You must know the following Table.

6 h11 p6 Family Functional Group haracteristic Alkene carbon-carbon double bond Alkyne carbon-carbon triple bond Aromatic aloalkane X six carbon atom ring with alternating single and double bonds carbon-halogen bond. X = F, l, Br, I Alcohol Ether carbon bonded to a hydroxyl group () oxygen atom bonded between two carbon atoms Thiol S carbon bonded to a S group Aldehyde Ketone carbonyl group (=) bonded to a hydrogen atom carbonyl group bonded to two carbon atoms arboxylic Acid carbonyl group bonded to a hydroxyl group Ester carbonyl group bonded to an oxygen atom Amine N Amide N carbon atom bonded to a nitrogen atom carbonyl group bonded to a nitrogen atom

7 h11 p7 Identify the functional group(s) in the following molecules: ethyl acetate 3 acetic acid 3 formaldehyde vanillin 2 N 3 N 2 N 2 trinitrotoluene 3 N insect repellant morphine NMe β-carotene l l 3 l dichloro-diphenyl-trichloroethane (DDT) l l l l 2,3,7,8-tetrachlorodibenzodioxin (dioxin) 3 3 N norethindrone capsaicin

8 h11 p8 As an example of the complexity of molecules that can be synthesized, consider Brevetoxin B below. Twelve years in the making, Brevetoxin B required 83 individual reactions in order to be synthesized. Me Me Me Me Me Me brevetoxin B Me It should be evident by the many different functional groups that the structure of the molecule is very important. onsider the molecules of ethyl alcohol and dimethyl ether. Although they have the same molecular formula, their molecular structure is clearly different. Note their different physical and chemical properties molecular formula molar mass 46 g/mol 46 g/mol room temperature liquid gas melting point boiling point reaction with Na vigorous none You will see that many molecules can have the same molecular formula but different arrangement of atoms.

9 h11 p9 This property is called isomerism. Molecules with identical molecular formulas but different connectivity of atoms are called constitutional isomers. Therefore, the two molecules shown above, ethyl alcohol and dimethyl ether are constitutional isomers. e.g. Indicate whether each of the following pairs of molecules represents identical compounds, constitutional isomers, or different compounds that are not constitutional isomers. a b c d. Draw all the constitutional isomers for 5 12.

10 h11 p10 ow many constitutional isomers can you draw for 4 8? Reactions of rganic ompounds. Approximately 90% of all organic reactions can be classified into three categories: 1. Addition Reactions. Molecules containing multiple bonds tend to undergo addition reactions. These include alkenes, alkynes, and molecules containing a carbonyl group. 2. Elimination Reactions. Molecules containing single bonds to heteroatoms tend to undergo elimination reactions. Elimination reactions are essentially the reverse of addition reactions. 3. Substitution Reactions. In substitution reactions, one atom/group on a molecule is replaced (substituted) by another atom/group. Most of the substitution reactions will see will involve alkanes and aromatic molecules.

11 h11 p11 Alkanes and ycloalkanes We begin our study of organic chemistry with the simplest class of compounds the hydrocarbons. As the name implies, hydrocarbons are compounds that contain only carbon and hydrogen atoms. The first class of hydrocarbons that we will study are the alkanes, which are characterized by carbon-carbon single bonds. The alkanes are also called saturated hydrocarbons because they cannot add any more hydrogen atoms to the structure. The general formula for alkanes is n 2n+2. e.g. atoms atoms Alkane 1 2(1)+2 = (2)+2 = (3)+2 = (10)+2 =

12 Nomenclature. h11 p12 IUPA International Union of Pure and Applied hemistry. The IUPA determines the protocol for naming organic compounds. The IUPA name of an organic compound can be divided among four fields as shown below. First field Second field Third field Fourth field substituents compound root saturation index principal functional group The first field includes the names and positions of substituents. The second field contains the compound root word. The third field indicated the saturation index (single or multiple bonds). The fourth field defines the principal functional group.

13 h11 p13 For naming continuous-chain alkanes, a prefix is used to describe the number of carbon atoms (compound root) followed by the suffix ane (saturation index). e.g. Alkane Prefix Suffix Name 4 meth- -ane methane 2 6 eth- -ane ethane 3 8 prop- -ane propane 4 10 but- -ane butane 5 12 pent- -ane pentane 6 14 hex- -ane hexane 7 16 hept- -ane heptane 8 18 oct- -ane octane 9 20 non- -ane nonane dec- -ane decane You must know these prefixes and names.

14 Summary of IUPA Rules for Nomenclature h11 p14 1. Find the principal functional group (PFG) in the molecule according to the priority ranking list. Note the suffix. 2. Find the longest carbon chain (or ring) that contains the PFG. This is the compound root word. 3. Number the chain or ring in such a way that the PFG is given the lowest possible number. Next priority is given to multiple bonds. If there is no PFG, number in a way to give the lowest series of numbers. If the series of numbers is the same, start with the substituent that comes first alphabetically. 4. The appropriate saturation index is inserted between the root word and the functional group suffix. If necessary, include a numeral to indicate at which carbon atom the multiple bond begins. 5. The substituents are specified by appending the appropriate prefixes and numerals to the. Substituents are listed alphabetically (ignoring any numerical prefixes and/or italicized words). 6. Stereochemical descriptors like cis-, trans-, (R)-, and (S)- are included as needed.

15 Functional Group Priority Ranking and Suffixes arboxylic acid (-oic acid) Ester (-oate) Amide (-amide) Aldehyde (-al) Ketone (-one) Alcohol (-ol) Thiol (-thiol) Amine (-amine) h11 p15 ***remember special names for benzene rings*** Substituent Prefixes - 3 methyl - 3 methoxy ethyl ethoxy propyl propoxy -( 3 ) 2 isopropyl -( 3 ) 2 isopropoxy butyl butoxy -( 3 ) 2 3 sec-butyl -( 3 ) 2 3 sec-butoxy - 2 ( 3 ) 2 isobutyl - 2 ( 3 ) 2 isobutoxy -( 3 ) 3 tert-butyl -( 3 ) 3 tert-butoxy -F fluoro = oxo -l chloro phenyl -Br bromo -I iodo - hydroxy -S mercapto 2 benzyl

16 Alkyl Groups in Branched ydrocarbons. h11 p16 So far we have seen how to name linear-chain alkanes but how do we name branched-chain alkanes? In the IUPA system, hydrocarbon substituents are named as alkyl groups. The alkyl group is named by replacing the ane ending of the corresponding alkane name with yl. Notice how the alkyl group has one less hydrogen atom than the alkane it is derived from. onsider the molecule below; it has two alkyl groups (positions 2 and 4) branched from the main chain of six carbon atoms The alkyl group at position 2 ( 3 ) is derived from methane ( 4 ) and so is named methyl. Likewise, the group at position 4 ( 3 2 ) is derived from ethane ( 3 3 ) and is named ethyl. Naming Branched-hain Alkanes. When using the IUPA system for naming hydrocarbons, the longest continuous chain or main chain contains the compound root word. The substituents are then numbered according to their position on the main chain.

17 Step 1. Find the longest continuous chain and name it as the compound root pentane h11 p17 Step 2. Identify the substituents on the main chain by their smallest position number. Use a prefix (di-, tri-, tetra-) to indicate a group that appears more than once. When multiple substituents allow numbering from both ends of the main chain, use the direction that gives the lowest series of numbers. If the series of numbers is the same, start with the substituent which comes first alphabetically. In the name, hyphens separate numbers from words and commas separate numbers methylpentane (not 4-methylpentane) ,3-dimethylpentane (not 3,4-dimethylpentane) ,2,4-trimethylpentane (not 2,4,4-trimethylpentane) ethyl-4-methylpentane (not 2-methyl-4-ethylpentane)

18 h11 p18 Step 3. When different substituents are present, list them in alphabetical order. The prefixes for repeated substituents are not used in deciding alphabetical order ethyl-2-methylpentane (not 2-methyl-3-ethylpentane) (not 3-ethyl-4-methylpentane) ethyl-2,4-dimethylheptane (not 2,4-dimethyl-3-ethylheptane) Give IUPA names for the following molecules

19 h11 p19 onformation of Alkanes. An important property of carbon-carbon single bonds is the rotation about this bond. Rotation about a carbon-carbon single bond gives rise to different arrangements of molecules called conformers. It is important to understand that conformers are the same molecule in different arrangements. onsider butane. Recall that compounds with the same molecular formula but with a different arrangement of atoms are called constitutional isomers. For example, for the formula 4 10 there are two possible arrangements: Remember; since each isomer has a different arrangement of atoms they have different properties. e.g. Draw all the constitutional isomers for hexane and give their IUPA names.

20 lassifying arbon Atoms in ydrocarbons. h11 p20 The carbon atoms in hydrocarbons can be classified according to the number of carbon atoms bonded to it. A primary carbon (1 ) is a carbon atom bonded to only one other carbon atom. A secondary carbon (2 ) is bonded to two other carbon atoms. A tertiary carbon (3 ) is bonded to three other carbons, and a quaternary carbon (4 ) has four bonds to other carbons. 1 carbon 1 carbons 4 carbon carbons 3 2 carbon 3 carbon lassify the carbons in the following molecules as primary, secondary, tertiary, or quaternary

21 Drawing Structures from IUPA Names. h11 p21 If you are given the IUPA name of a compound you have all the information necessary to draw its structure. Start by identifying the compound root name, and then add the substituents at the indicated positions. onsider 2,3- dimethylhexane. The name can be dissected into its individual components. substituent positions substituent saturation index 2,3 di methyl hex ane number of similar substituents compound root Draw the condensed structure for 4-ethyl-2,2- dimethyloctane. ycloalkanes. Up to now we have only considered straight-chained or branched-chained saturated hydrocarbons. f course, alkanes can also form cyclic structures called

22 h11 p22 cycloalkanes. ycloalkanes, with general formula n 2n, have two fewer hydrogen atoms than the corresponding alkanes. Naming cycloalkanes is similar to straight-chain alkanes except that the prefix cyclo- is added to the name of the alkane. When one substituent is present, the substituent name is placed before the cycloalkane name. No number is needed for one substituent. When two or more substituents are present, the numbering starts by assigning carbon 1 to the substituent that gives the lowest series of numbers ethyl-4-methylcyclooctane not 1-methyl-4-ethylcyclooctane nor 1-ethyl-6-methylcyclooctane 3 methylcyclobutane ,3-dimethylcyclohexane not 1,5-dimethylcyclohexane

23 h11 p23 Since the cycloalkanes are in a cyclic arrangement, rotation about the carbon-carbon single bonds within the ring is restricted. This gives the cycloalkanes two distinct sides or faces. As a consequence, this interesting characteristic gives rise to cis-trans isomers. is-trans isomers only differ in the orientation of atoms in space. onsider the molecule 1,2-dichlorocyclopropane. The structure can be written with the chlorine atoms on the same side (cis isomer) or with the chlorine atoms on opposite sides (trans isomer). Look at the molecules carefully. onvince yourself that the two isomers represent different molecules. l l l l cis-1,2-dichlorocyclopropane trans-1,2-dichlorocyclopropane Physical Properties of Alkanes and ycloalkanes. Alkanes and cycloalkanes are nonpolar (why?) and therefore insoluble in polar solvents such as water. They typically have densities around 0.6 to 0.7 g/ml which is lower than that of water 1.0 g/ml. This is why oil will float on top of water. Since all the atoms in alkanes are carbon and hydrogen (which have very close electronegativity values), small

24 h11 p24 dipoles are generated throughout the molecule which result in very weak intermolecular forces. As the number of carbon atoms increase, the amount of intermolecular forces also increases. Although these forces are relatively weak, the cumulative effect of these intermolecular forces does lead to a gradual progression through the different states of matter. 1-4 carbons gases 5-17 carbons liquids 18 and greater solids (greases, waxes, asphalt) The boiling points for branched-chained alkanes tend to be lower than straight-chained alkanes with the same number of carbons. This is because branched-chain alkanes are more compact which reduces the amount of intermolecular forces (due to less surface area). ycloalkanes on the other hand, have slightly higher boiling points. Because rotation of the carbon-carbon bonds is restricted, cycloalkanes are more rigid than the straight- or branched-chained alkanes. This rigidity allows the cycloalkanes to stack together and therefore increase the amount of intermolecular forces between molecules. The alkanes are invaluable to our everyday life. As shown above, the first four alkanes (methane, ethane, propane, and butane) are gasses and are used extensively as heating fuels. Alkanes having 5-8 carbon atoms are

25 h11 p25 liquids and are used in fuels such as gasoline. Alkanes containing 9-17 carbon atoms are also liquids but have higher boiling points. This makes them useful in kerosene, diesel, and jet fuels. Alkanes with 18 or more carbon atoms are waxy solids at room temperature. These hydrocarbons, known as paraffins, are used to make waxes, petroleum jelly (Vaseline) and other ointments. The different alkanes are typically obtained from crude oil. rude oil is found as a mixture of different hydrocarbons which is refined by fractional distillation, from which many useful products are made. hemical Properties of Alkanes and ycloalkanes. Because alkanes are made up of carbon-carbon single bonds, which are difficult to break, they are the least reactive family of organic compounds. owever, this does not mean that they are inert. As evident from their extensive use as fuels, saturated hydrocarbons react (burn) readily in oxygen. They also undergo reactions with the halogens (typically with l and Br). You have already seen the combustion reaction in hapter 6. Alkanes undergo combustion when they react with 2 to form 2, 2, and heat. The general reaction is shown below:

26 alkane heat h11 p26 The combustion of alkanes is not always complete however. If the supply of oxygen is insufficient, carbon monoxide () will form instead of carbon dioxide ( 2 ). arbon monoxide is a colourless, odourless, poisonous gas and can be lethal if produced in a non ventilated area. is poisonous because it binds to hemoglobin stronger than oxygen. emoglobin is responsible for transporting oxygen to our cells and organs. Therefore, if the concentration of is high enough in a person, that person will suffocate. The incomplete combustion of methane is shown below: heat aloalkanes. A subclass of the alkanes is the haloalkanes. aloalkanes are alkanes in which one or more hydrogen atoms are replaced by halogen atoms. The halogens are named as substituents: alogen Name fluorine chlorine bromine iodine fluorochlorobromoiodo-

27 h11 p27 Simple haloalkanes still use their common names, some of which do not indicate their structure. They are named as alkyl halides. Some examples are shown below. IUPA ommon 3 l chloromethane methyl chloride 3 2 Br bromoethane ethyl bromide F fluoropropane isopropyl fluoride l l l dichloromethane methylene chloride l l trichloromethane chloroform l l l l tetrachloromethane carbon tetrachloride

28 h11 p28 alothane, F 3 lbr, is a widely used general anesthetic. What is its IUPA name? Synthesis and Reactions of aloalkanes. Beside combustion reactions, alkanes can also undergo free radical halogenation reactions. The reaction of an alkane with a halogen is called a substitution reaction because the reaction involves the replacement of one or more hydrogen atoms with halogen atoms (compare with replacement reaction). These reactions are done in the presence of ultraviolet light. + l l hν l + l In the presence of excess light and chlorine, the reaction will continue until all the hydrogen atoms are replaced. l l l l l 2 l 2 l 2 l l l l + l l + l l + l

29 h11 p29 With longer chain alkanes, any of the hydrogen atoms can be substituted giving rise to many isomers. owever, because 3 hydrogens are more reactive than 2 hydrogens which are more reactive than 1 hydrogens, the product distribution can be unequal. 2 carbon Br Br Br + Br 60% 40% 1 carbon Most of you are familiar with haloalkanes. hlorofluorocarbons (Fs) are haloalkanes which are used as propellants and refrigerants. Two common Fs, Freon 11 and Freon 12 are shown below. l l l F l Freon 11 Freon 12 Fs are non toxic, non flammable, unreactive, and extremely robust. owever, in the mid 70s, F. S. Rowland and M. J. Molina showed that Fs were responsible for the depletion of the ozone layer. They showed that in the stratosphere, Fs broke down to give chlorine radicals (l ) which started a chain reaction. F F l

30 h11 p30 These radicals reacted with ozone molecules ( 3 ) to give oxygen molecules. More importantly, however, the chlorine radicals were regenerated and started the whole process again. It has been estimated that one chlorine radical can destroy as many as 100,000 ozone molecules. The reactions are shown below using Freon 12 as an example. hain initiation l l hν Step 1 F l F + F F l (chlorine radical) hain propagation Step 2 l + 3 l + 2 (ozone) Step3 l + 3 l + 2 2

31 Important oncepts from hapter 11 Structures of rganic ompounds Expanded ondensed Line-Bond h11 p31 Isomers onstitutional Isomers rganic Reactions Addition Reactions Elimination Reactions Substitution Reactions Functional Groups. IUPA System for Nomenclature onformation of Alkanes arbon Atom lassification 1, 2, 3, and 4. Reactivity of Alkanes aloalkanes Reactivity of aloalkanes