Chapter 22 Hydrocarbon Compounds

Transcription

1 Chapter 22 Hydrocarbon Compounds 1 ORGANIC COMPOUNDS Organic compounds are carbon compounds and there are over a million. The simplest organic compounds are hydrocarbons and they are composed of hydrogen bonded to carbons. Ex. methane and ethane. Carbon can form so many different compounds because it has four valence electrons and will always form four covalent bonds. These four bonds can involve single, double, or triple bonds, just as long as there are four of them in total. Organic compounds can be divided into two types: aliphatic molecules and aromatic molecules. Aliphatic molecules can be cyclic molecules (rings) while others are chain molecules (non-cyclic). Aromatic compounds will be described further on page 3. In organic chemistry, the number of carbons is indicated using an IUPAC prefix system. Numbers 1 to 10, respectively, have the following prefixes: meth- eth- prop- but- pent- hex- hept- oct- non- dec-. The highlighted ones are the only ones that are different from the ones used to name all other molecules. There are different ways that structural formulas (aka structural diagrams) for organic compounds can be drawn. Their molecular formula (aka chemical formula) does not share as much information about bonding as structural formulas do. This will be discussed using examples throughout the material. A family of hydrocarbons with similar chemical properties and who share the same general formula are called a homologous series. The following three homologous series are all aliphatic hydrocarbons. ALKANES All of the carbons in an alkane are bonded to either other carbons or hydrogens AND all of the bonds are single bonds. The bonding pair of electrons in a C-H or C-C bond essentially nonpolar, and as such, alkanes are nonpolar molecules that tend to rely solely on dispersion forces to attract one another. For this reason, the smallest alkanes are gases, and those that are slightly larger (which still have a low molar mass) tend to be liquids or gases that boil at low temperatures. Since alkanes are nonpolar, they will not dissolve in water. As seen in Fig. 22.1, the names of all alkanes begin with the prefixes mentioned above and end with ane (ex. methane, ethane, propane, butane, etc.). The carbon atoms in an alkane can be arranged in (1) a straight chain or (2) a chain with branches. (1) Straight-chain alkanes don t involve any branching. In Fig. 22.1, notice the boiling points increase as the number of carbon atoms increase. The change reflects the increase in molar mass and the intensity of the dispersion forces that exist. In Fig. 22.2, notice the different structural formulas that can be drawn to better represent the molecular structure of an alkane like butane. Conceptual Problem 22.1, Practice Problems #1 and 2. p. 697

2 (2) Branched-chain alkanes discuss branches as if they were substituted for a hydrogen atom on the chain. The parent chain is the longest continuous carbon chain and the substituent is an atom or group of atoms (like in this case) that takes the place of a hydrogen atom. 2 A hydrocarbon substituent (ie. a branch) is called an alkyl group. They can be one carbon or several carbons long. They are named by taking the appropriate prefix and adding yl (methyl, ethyl, propyl, etc.). When you place an alkyl into a straight-chain hydrocarbon, branches are formed. An alkane with one or more alkyl groups (branches) is called a branched-chain alkane. Naming these structures has several easy steps: 1. Find the longest chain of carbons in the molecule. 2. Number the carbons in the main chain in sequence. Start at the end that would give your atoms with branches the lower numbers. 3. Add numbers to the names of the branches to help keep track of their position. (ex: 2-ethyl, 3- methyl, etc) 4. Use prefixes to indicate the same groups that appear more than once. 5. List the names of alkyl substituents in alphabetical order (ignoring the prefixes). 6. Use the proper punctuation.commas to separate numbers, hyphens to separate numbers and words, no spaces. Conceptual Problem 22.2, Practice Problems #3 and 4. p.699 To go in the opposite direction, from an alkane name to the structural formula, the following steps apply: 1. Find the root word (ending in ane). Use this to write the parent molecule. 2. Number the carbons on this parent chain. 3. Identify the substituent groups and attach them to the numbered parent chain. 4. Add hydrogens as needed. Conceptual Problem 22.3, Practice Problems #5 and 6. p. 700 ALKENES Organic compounds that contain the maximum number of hydrogen atoms per carbon atom are referred to as being saturated. All alkanes are saturated compounds as the only bonds within them are single covalent bonds. Compounds with double or triple carbon-carbon bonds are classified as being unsaturated. Alkenes are hydrocarbons that contain at least one carbon-carbon double bond. Alkenes are named by the length of the longest carbon parent chain and by adding ene (ex. ethane, propene, butene, etc.). ALKYNES An alkyne is another type of unsaturated hydrocarbon that contains at least one carbon-carbon triple bond. Alkynes are named by the length of the longest carbon parent chain and by adding yne (ex. ethyne, propyne, butyne). To properly name alkenes and alkynes, the numbering of the carbons must give the carbon with the unsaturated bond the lowest number possible as seen in bottom right corner of the figure above. Practice will be done in class using other sources.

3 ISOMERS Compounds that have the same molecular formulas but different molecular structures are called isomers. There are two types of isomers: (1) structural isomers and (2) stereoisomers. (1) Structural isomers are compounds that have the same molecular formula but the atoms are joined in a different order (ex. butane vs. 2-methylpropane). They differ in physical properties such as melting and boiling points. The more highly branched the hydrocarbon structure, the lower the boiling point of the isomer. Their chemical reactivity would be different. (2) Stereoisomers involve atoms joined in the same order, but the position of the atom is different. There are two types of stereoisomers: (a) geometric isomers and (b) optical isomers. (a) Geometric isomers are based on the presence of a double bond in a molecule. The double bond between two carbon atoms prevents them from rotating with respect to each other and because of this, groups on either side of the double bond can have different positioning in the space they occupy. There are two possible orientations: (i) trans and (ii) cis. (i) Trans: the substituents are on opposite sides of the double bond. (ii) Cis: the atoms are on the same side of the double bond. 3 (b) Optical isomers occur whenever a carbon atom has four different atoms or groups attached to it (an asymmetric carbon). Optical isomers are the mirror image of one another. Conceptual Problem 22.4, Practice Problems #18 and 19. p. 706 CYCLIC HYDROCARBONS Compounds that contain a hydrocarbon ring are called cyclic hydrocarbons. Rings with five or six carbons are the most abundant, but rings containing up to twenty carbons can be found in nature. Organic compounds that are responsible for the aromas of spices such as vanilla, cinnamon, cloves, and ginger are called aromatic compounds (although not all aromatic compounds have an odor). They are stable, unsaturated cyclic compounds. They contain multiple double bonds that are special because they are not fixed in one place. The electrons making up these bonds are delocalized and so can move around in an organized fashion, allowing for double bonds to alternate with single bonds. Resonance is the term referring to the phenomenon of delocalization, or having more than one acceptable bond structure. Sometimes the ring structure may include a nitrogen, oxygen, or sulfur. Benzene is an aromatic six-membered carbon ring with a hydrogen atom attached to each carbon and it is the most well-known aromatic compound. Benzene is not as reactive as six-carbon alkenes.

4 Substituted aromatic compounds contain substituents attached to a benzene ring are known as derivatives of benzene. They are often used as dyes to produce strong color. 4 Disubstituted benzenes are derivatives of benzene that have two substituents. They are called dimethylbenzene (C6H4(CH3)2) and there are three structural isomers for this liquid aromatic compound. They can also be called xylenes. The possible positions of two substituents are designated as 1,2; 1,3; or 1,4. Terms for the disubstituted benzenes include, ortho, meta, and para. Chapter 23 Introduction to Functional Groups FUNCTIONAL GROUPS In most chemical reactions involving organic molecules, the saturated hydrocarbon skeleton of the molecules are chemically inert. Most organic chemistry is actually functional-group chemistry. A functional group is a specific arrangement of atoms in an organic compound that is capable of chemical characteristic reactions. Often functional groups contain O, N, S, and/or P, but double and triple bonds are also chemically reactive and so they are also considered functional groups. Organic compounds can be classified according to their functional groups. Table 23.1 nicely organizes nine different functional groups (excluding double and triple bonds). Note that R represents a hydrocarbon chain or ring. HALOGEN SUBSTITUENTS Halocarbons contain covalently bonded F, Cl, Br, or I. They have both IUPAC names and common names (in brackets) as seen in the image on the right. If you look at the format of the common names, you can see that alkyl halides are aliphatic hydrocarbons with a halogen attached and that aryl halides are arenes (aromatic rings) that have a halogen attached. at

5 The more halogenated the halocarbon is (more halogen substituents), the stronger the intermolecular attractions and the higher the boiling points. Halocarbons are rarely found in nature but they can readily be prepared. 5 SUBSTITUTION REACTIONS Since relatively strong covalent bonds exist within organic compounds, organic reactions are usually slower than inorganic reactions and require catalysts. A substitution reaction involves atoms being replaced by other atoms, such as a halogen replacing a hydrogen and forming a halocarbon (aka halogenation). * Note that many organic reactions are General format complex and often produce a mixture of products. This halogenation would actually produce mono-, di-, tri-, and tetrachloromethanes! * Iron compounds are often used as catalysts in aromatic substitution reactions. General reaction * Halogens are readily displaced by hydroxide ions to produce and alcohol and salt. * Fluoro groups are not easily replaced so they would rarely be used to make alcohols. ALCOHOLS Alcohols have an OH group. This is a functional group called a hydroxyl group (aka hydroxy function). Aliphatic alcohols can be classified into structural categories (primary, secondary, and tertiary alcohols) according to the number of R groups attached to the carbon with the hydroxyl group as seen on the right. Both IUPAC and common names are used for alcohols. According to the IUPAC system, the parent alkane will end in ol and the numbering of the carbons gives the hydroxyl group the lowest number possible. If there is more than one hydroxyl group (glycols), they are called diols, triols, tetrols, etc. Notice the name of the parent chain below when there is only one hydroxyl group compared to when there is more than one. Alcohols are capable of hydrogen bonding since hydrogen is bonded to a very electronegative atom (the oxygen). Because of this strong intermolecular attraction, alcohols boil at higher temperatures than comparable alkanes and halocarbons. The polarity of the O-H bond also make alcohols up to four carbons quite soluble in water; usually if an alcohol has more than four carbons the solubility is much lower because the saturated carbon chain is highly nonpolar.

6 ADDITION REACTIONS In an addition reaction a substance is added at the double or triple bond of an unsaturated hydrocarbon. Addition reactions of alkenes are an important method of introducing new functional groups into organic molecules. General format 6 * The addition of water to an alkene is a hydration reaction. They are often catalyzed by a strong acid at 100 C. * Notice here that the addition of a halogen to an unsaturated hydrocarbon would result in a disubstituted halocarbon. * Hydrogen halides being added to a double bond result in a monosubstituted halocarbon. * Hydrogenation reactions add hydrogen to a double bond and produce an alkane. They are usually catalyzed by Pt or Pd. * Benzene resists hydrogenation, the addition of a halogen, and the addition of a hydrogen halide unless under the conditions of high temperatures and pressures and in the presence of a catalyst. ELIMINATION REACTIONS During an elimination reaction, an alkyl halide reacts with a hydroxide ion to produce and alkene by removing a hydrogen (resulting in H2O with hydroxide) and a halide ion.

7 COMBUSTION Complete combustion of hydrocarbons always results in the formation of carbon dioxide and water because there an excess of oxygen present for the reaction. Incomplete combustion of hydrocarbons occurs when there isn t enough oxygen present to react so carbon and carbon monoxide will form as a result. 7 CRACKING Cracking is a process involving complex organic compounds or long-chain hydrocarbons and breaking them down into simple molecules by breaking carbon-carbon bonds. This process is largely dependent on the temperature and the presence of catalysts. ETHERS The general structure of an ether is R O R; it is a compound in which oxygen is bonded to two carbon groups. The alkyl groups attached to the ether linkage are named in alphabetical order and are followed by the word ether. Symmetric ethers (the same R group) are named using the prefix di-, even though sometimes the di- is dropped (ex. diethyl ether is simply ethyl ether. Ethers can form hydrogen bonds because the oxygen has lone pairs that strongly attract hydrogens on a neighboring molecule if the hydrogen is part of a strong polar covalent bond. That said, although they can form stronger intermolecular attractions than hydrocarbons and halocarbons, ethers cannot form as many polar bonds as an alcohol. Consequently, ethers have higher boiling points that hydro- and halocarbons but lower boing points than alcohols. Also, ethers are more soluble in water than hydro- and halocarbons but less soluble in water than alcohols. ALDEHYDES AND KEYTONES When oxygen is bonded to a carbon by a double covalent bond, it is called a carbonyl group(c=o). There are two types of carbonyl compounds: (1) An aldehyde has the oxygen on a terminal carbon (general formula RCHO), and (2) a keytone has the oxygen bonded to any of the carbons that are in the middle of the molecule (general formula RCOR). Aldehydes and keytones are named by identifying the longest chain containing the carbonyl group. Aldehydes replace the e ending with al and the carbonyl group designates the first carbon. Keytones

8 end with one instead (sounds like phone ) and if there is more than one possible carbon for the carbonyl group to go, its position is designated by the lowest possible number. 8 Aldehydes and keytones can form weak hydrogen bonds with water, however their solubility in water decreases as a result of longer carbon chains; in other words, when the carbon chain of an aldehyde or keytone exceeds five carbons, their solubility in water is very low. Aldehydes and keytones can attract each other through dipole interactions because of their carbonyl group which makes them have higher boiling points than corresponding alkanes but lower boiling points than corresponding alcohols. It is because of the dipole interactions that almost all aldehydes and keytones are liquids or solids at room temperature (except methanal, which is a gas). Aromatic aldehydes are often used as flavoring agents (ex. benzaldehyde and cinnamaldehyde). CARBOXYLIC ACIDS A carboxylic acid (general formula RCOOH) contains a carboxyl group a carbonyl group attached to a hydroxyl group. They are weak acids that poorly ionize in water to form a carboxylate ion and a proton. The IUPAC system names carboxylic acids by dropping the e of the parent alkane and adding oic acid. Many continuous-chain carboxylic acids were first isolated from fats and are called fatty acids. Common names are used more often than IUPAC names for fatty acids. Small aliphatic carboxylic acids are colorless, volatile liquids with unpleasant odors, whereas larger ones are nonvolatile, odorless, waxy solids that melt at low temperatures. All aromatic carboxylic acids are crystalline solids at room temperature. Carboxylic acids can form intermolecular hydrogen bonds and therefore have higher melting and boiling points than other compounds of similar size. The carboxyl group is polar and will also form hydrogen bonds with water, making smaller carboxylic acids highly soluble (miscible) in water. ESTERS Esters are derivatives of carboxylic acids in which the OH has been replaced by an OR from an alcohol. They contain a carbonyl group and an ether link to the carbonyl carbon (general formula RCOOR). Many esters have pleasant, fruity odors. Esters are named as if the alkyl chain from the alcohol is a substituent. No number is assigned to this alkyl chain. This is followed by the name of the parent chain from the carboxylic acid part of the ester with an e remove and replaced with the ending oate. Esters form weak intermolecular attractions and therefore have lower boiling points than the carboxylic acids from which they are derived. Esters containing more than four or five carbons have limited solubility. Esters may be prepared from a carboxylic acid and an alcohol in a process called esterification.

9 POLYMERIZATION Some of the most vital organic compounds are giant molecules called polymers (ex. plastics). A polymer is formed by covalent bonding of repeating smaller units called monomers. Polymers can contain the same monomer or different types of monomers. The joining of monomers is called polymerization and most polymerization reactions require a catalyst. An addition polymer forms when unsaturated monomers react to form a polymer. 9 Polyethylene, a key component of plastic, exhibits different physical properties based on how long of a polymer it is (ie. a shorter carbon chain results in a paraffin wax consistency and a longer carbon chain leads to a harder and more rigid product). Condensation polymers are formed by the head-to-tail joining of monomer units and usually the formation of water (ex. the formation of a polyester in the diagram). Polymerization requires that there be two functional groups on each monomer molecule. Note that in the example below, (1) the functional groups are a carboxyl group and hydroxyl group, and (2) a block diagram is used with the squares and circles representing unreactive parts of the organic molecules. Other condensation reactions may involve a carboxylic acid and an amine (general formula RNH2), which has an amino group (-NH2). The resulting linkage involves an amide bond which makes the molecule an amide (ex. nylon). Proteins, which are polyamides if naturally occurring amino acids, rank among the most important of all biological molecules.