Hydrocarbons the Study of Nomenclature, Structure and Isomers

HYDROCARBONS THE STUDY OF NOMENCLATURE, STRUCTURE AND ISOMERS

• PURPOSE. each carbon must have four bonds. When these bonds are all single bonds, they are sp3 hy To gain an understanding of the various structural bridized-that is, all four bonds are equivalent. The aspects of hydrocarbons. geometric shape of a sp3 hybridized carbon is tetra hedral. When carbon is double or triple bonded to another carbon atom, the hybridization changes. In the case of double bonds, the carbon becomes sp2 hybridized and displays a trigonal planar geometry. The geometrical shape becomes linear for triple bonded, sp-hybridized carbon atoms. OBJECTIVES Methane, CH4, is the simplest hydrocarbon. When you build methane with the model kit, you 1. To name a hydrocarbon when given its structural will be able to see it on a 3-D basis. But to represent formula. this structure on paper in 3-D takes practice. The wedge bond represents a bond to an atom directed 2. To draw the structure of a hydrocarbon when in front of the paper, while the dashed bond indi given the name. cates a bond directed behind the paper. It is much 3. To manipulate the parts of a chemistry model kit easier to flatten out the model as shown below. in order to visualize the three-dimensional (3-D) This is called a structural formula because it structure of molecules. shows all the individual bonds. A further simplifi 4. To identify and formulate structural isomers. cation of the structural formula is known as the 5. To identify and formulate geometric isomers. condensed formula of a compound. In this ver sion, the number of hydrogen atoms are summa rized with the use of a subscript.

H H I H/4", I CH MATERIALS AND CHEMICALS ·c H-C-H 4 Hf'" 'H I H • Model kits 3-D structural structural condensed

The names of hydrocarbons basically consist of three parts; prefix, root, and suffix. The root indi TIME REQUIRED cates the number of carbons in the parent chain, the longest chain of carbons. These are listed in Table l. 1-2 hours Memorize them. The ending of the name indicates the type of hydrocarbon. If the compound is a saturated hydro carbon, it possesses only single bonds, and it is BACKGROUND called an alkane. The suffix on alkanes is -ane. If the hydrocarbon has at least one carbon-carbon double Hydrocarbons represent the simplest group of or bond, it is termed an alkene and ends in -ene. ganic compounds since they are made up of only Alkynes, hydrocarbons with at least one triple carbon and hydrogen. In any organic compound, bond, end with -yne.

205 Exploring Chemistry

Table 1 Nomenclature roots The substituent, CHJ CHCH3, is called the iso 1 meth- 6 hex- propyl group. Other substituents that should be 2 eth- 7 hept- readily identified are shown below. Memorize these. 3 prop- 8 oct- 4 but- 9 non- n-propyl S pent- 10 dec-

If an alkyl substituent is present on the par isopropyl ent chain, it is designated with a -yl ending. The name and position (by number) of the sub stituents make up the prefix of the hydrocarbon terl-butyl name. The longest continuous chain, the parent chain, is first determined. Numbering of the longest chain ensures that the substituent(s) re ceive the smallest number(s) possible. Let's look sec-butyl at some examples where we draw the structural formula when given the name of the compound. Example 4

Example 1 eH, 3 pentane Q-CHCH2CH3 / I \ no prefix root: suffix: alkane: One correct nanH:' for the above compound is (no substituents) 5 carbons all single bonds sec-butylcyclohexane. The IUPAC name is 1- methylpropylcylohexane. Notice that this example structural formula: also demonstrates the nomenclature for a cyclic hy H H H H H drocarbon. Carbons in the ring are found at the in I I I I I tersections of the lines and the hydrogen atoms are H-C-C-C-C-C-H I I I I I not shown, but are understood. The prefix cydo- is H H H H H used to indicate a hydrocarbon ring structure.

Example 2 2-mcthylbutane o cyclooctane / I \ Example 5 prefix: methyl root: 4 carbon suffix: 2,3-dimethylhexane group on parent chain alkane carbon 2 structural formula: H H , I H-C-H H-C-H 1 I f,l 1 1 1 III I 1 1 H-C-C-C-C-C-C-H H-C-C-C-C-H , I I I , , I I I I H H H H H H H H H H-C-H, H Example 3 condensed formula: CH 4-isopropylheptanc , .1

C1l 1CH 7CH,CHCIhCH 2CH3 CH 3CH?HCH2CH 2CH 3 . - ~, - .

CH3CHCH3 CH)

206 Hydrocarbons This last example brings up the next topic Are the two structures drawn above isomers isomerization. Structural isomers, also known as or just the same compound? constitutional isomers, are compounds that have For unsaturated compounds, the position of the same number of atoms, but are arranged in a the double or triple bond needs to be expressed. different order. 2,3-Dimethylhexane is just one ex Furthermore, when drawing these structures, keep ample of the many isomers of octane, CSH 1S' in mind that carbon can only have four bonds. Also, double and triple bonds must be shown in the con densed formulas of alkenes and alkynes.

Example 6

Let's also look at hexane, C6H 14. 2-Methylpen H H H H H tane and 3-methylpentane are both isomers of hexa I I I I I H-C-C=C-C-C-H ne. Look at their condensed formulas. I I I Is the following compound another isomer H H H of hexane? 2-pentene

The 2- indicates that the double bond begins at carbon no. 2. Now, look at the condensed formula.

Actually, it isn't. This compound is the same as 2-methylpentane just flipped end for end. There fore, one way to check if two compounds are actual ly isomers is to correctly name them. If the two Does 2-pentene have a structural isomer? compounds contain the same number of atoms Most alkenes can have another type of isomer (same molecular formula), but have different called a stereoisomer. These isomers exist because names, then they are isomers. of the fact that there is restricted rotation around the As compounds increase in length, the number carbon-carbon double bond. A double bond does of possible isomers increases. A systematic method of not permit free rotation. So we define stereoisomers writing all the isomers for a given hydrocarbon (or as compounds that contain the same number of any other organic molecule, for that matter) can be atoms, the same atom-to-atom attachments, but dif utilized. The first structure is the straight chain with ferent geometric arrangements. In the case of no branches. Then construct a chain with one less double-bonded compounds, this is called cis-trans carbon atom and a methyl group attached in as many (or E, Z) isomerism. We will use 2-pentene again as places as possible. Again, shorten the carbon chain our example. and try two methyl group branches. Continue with more methyl side chains until it is no longer possible. H H CH3 H Next, use one ethyl side chain. (Remember to C=C/ check your total number of carbons!) Put the ethyl "C=C / " / side chain in as many places as possible. Then try a CH( "CH2CH3 H "CH 2CH, propyl, isopropyl, butyl, and so on. Watch that your cis-2-pentene trans-2-pentene side chains do not contain a longer carbon chain than (Z)-2-pentene ( E)-2-butene your parent chain. Finally, you might also try combi nations of a methyl and ethyl group. Name each of your isomers to be sure you have not duplicated a structure. When you build these two isomers with your model kit, you should prove to yourself that they are not the same compound. They differ in their arrangement in space. The "cis" [or (Z)] in dicates that the carbon groups are on the same side, whereas the "trans" [or (E)] means that they CH.1 -CH-CHI 2 -CH-3 are on opposite sides of the double bond. Geo metric isomers also differ in many of their physi CH2 cal properties, such as melting point, boiling I point, and color. CH3

207 208 Name ______Date

Laboratory Instructor ______

PRE-LABORATORY ASSIGNMENT Hydrocarbons

1. Define the following:

a. Hydrocarbon

b. Structural formula

c. Condensed formula

d. Structural isomers

e. Geometric isomers.

2. Draw the structural formulas for C2H 6 and C3Ha.

3. Draw the condensed formulas of 3-hexene and 2-methylbutane.

4. Draw the structural formula for 3-pentyne. What is wrong with this name?

209 210 Name ______Date

Laboratory Instructor ______

STUDENT REPORT Hydrocarbons

A. Molecular Models Experiment

In the kit, you will find wooden balls representing atoms. They are color coded: carbon, black; hydrogen, yellow; nitrogen, blue; oxygen, red; chlorine, green; bromine, orange; iodine, purple. The number of holes drilled into each wooden ball represents the number of covalent bonds that must be formed by that atom. The location of the holes represents the actual angle formed when bonding. You will also find springs and two sizes of wooden pegs, The short pegs are for bonds between carbon and hydrogen. The long pegs should be used between carbon atoms. Multiple bonds can be represented using the springs. For a double bond, two springs are needed. For a triple bond, three springs are used,

1. Alkanes: build methane, butane, the isomer of butane, and pentane,

a. Draw the structural and condensed formulas for each of the following compounds.

Name Structural formula Condensed formula

Methane

Ethane

Propane

Butane

Butane's isomer

Pentane

211 Exploring Chemistry 2. Alkenes: build ethene, 2-butene, and 4-ethyl-2-hexene a. Draw the structural and condensed formulas for each of the following compounds.

Name Structural formula Condensed formula

Ethene

Propene

2-Butene

4-Ethyl-2-hexene

3-lsobutyl-l, 4-pentadiene

Did you notice the lack of rotation around the double bond?

b. Which of the above compounds possess geometric isomers?

c. Draw a structural isomer for 2-butene. Also, draw a structural isomer forl-C'thyl-2-hexene.

3. Alkynes: build ethyne and 1,3-butadiyne a. Draw the structural and condensed formulas for each of the following compounds.

Name Structural formula Condensed formula

Ethyne

(continued)

212 Hydrocarbons (continued)

Name Structural formula Condensed formula

Propyne

1,3-Butadiyne

3-Methyl-l-pentyne

Does each carbon have 4 (and only 4) bonds?

B. Reflective Exercises -I-hre..e. 1. Draw the ~~isomers of pentane, CSHlu both structural and condensed formulas, and name them.

2. Name the following compounds.

a. b.

CH CH?CHCH 3 _I 3

CH2CH3

c. d.

213 Exploring Chemistry

3. Draw and name the 18 structural isomers of CSH I8. (If necessary, use additional paper.)

4. Name the following compounds.

a. CH I 3 CH -CH-CH=C-CH J I 3

CH2CH3

c. CH CH CH I 3 I 2 ] CH1--C-CH?-CH-CHI - .1

CH2CH3

5. Draw the structure of 1-ethyl-2,2-dimethylcyclohexane.

6. Draw four structural isomers that have the formula C4 H9Cl.

7. Write the two geometric isomers of the alkene having the molecular formula C3 H 4 C1 2-

214 REACTIVITY OF HYDROCARBONS

• PURPOSE. BACKGROUND

To explore experimental tests of several hydrocar Hydrocarbons, as the name implies, are compounds bon compounds. made up of hydrogen and carbon. These com pounds are found plentifully in our high technolog ical society. Some examples include fuels, paints and paint thinners, pesticides, plastics, and cleaning solvents. If you own a gas grill, the simple hydro carbon propane is used as the fuel. Low-molecular-weight hydrocarbons are OBJECTIVES found in the gaseous state. Many hydrocarbons, such as octane, are liquids. Higher molecular 1. To predict the product(s) when a hydrocarbon weight hydrocarbons are found in the solid state. reacts with an oxidizing agent. These are often termed paraffins. 2. To predict the product(s) when a hydrocarbon Hydrocarbons can be classified into two main reacts with the bromine reagent. groups: aliphatic and aromatic hydrocarbons. 3. To predict the products of a hydrocarbon com Aliphatic compounds do not contain any aromatic bustion reaction. rings. This group can further be broken down into saturated and unsaturated hydrocarbons, which dif 4. To predict the solubility of a hydrocarbon with fer greatly in their reactivity. Unsaturated hydrocar another compound. bons are those that contain at least one double or triple 5. To discuss the volatility of hydrocarbons. bond. Scheme 1 summarizes these groupings. 6. To predict the reactivity of household hydro carbons with potassium permanganate. PHYSICAL PROPERTIES 7. To describe the hazards associated with hy The solubility behavior of hydrocarbons leads to drocarbons. an understanding of their waste disposal. Hydro carbons are nonpolar substances since they are composed of only carbon and hydrogen. The elec trons in a C-H bond (as well as a C-C bond) MATERIALS AND CHEMICALS are shared quite equally among the two atoms.

• Spot plate, small test tubes, stirring rod, UV lamp • Hexane, pentane, heptane, cyclohexene, toluene, HYDROCARBONS bromine reagent, KMn04 reagent, gasoline, paint thinner, Vaseline, mineral oil, unknowns ~ • 2 labeled waste jars AROMATIC ~

UNSATURATED: SATURATED: ~ alkanes (C-C) TIME REQUIRED all single bonds o alkenes (C=C) alkynes (C=C) double bonds triple bonds 1-1 hours Schemel

215 Exploring Chemistry Therefore, no distribution of charges exists; no par REACTIVITY tial positive or partial negative charges are created. Water, 011 the other hand, is a very polar com Burning pound. Since the oxygen has a greater affinity for the electron pair in an 0-H bond than the hydro The various groups of hydrocarbons differ in their g('I1, a polar bond is generated. reactivities. Saturated hydrocarbons are very stable As a rule of thumb, like dissolves like. A nonpolar substances, although in the presence of oxygen they compound will mix well with, or dissolve, another burn quite efficiently; this explains their ability to nonpolar compound. The same situation applies to be used as fuels. The burning reaction yields C02 pplar compounds. It should then be obvious that hy and H20, but undesirable products such as CO and drocarbons do not mix with water. Consequently, free carbon may also form. The following reaction is many organic substances can be considered "prob an example of the burning process. lematic" to sewage treatment plants and to septic sys tems. These are the substances that are not "sewpr compatible" and contaminate the water supply. Ele vated levels of hydrocarbon solvents have been de tected in drinking water where abnormally high Bromination incidences of leukemia have been reported. Laws and A hydrogen bonded to a carbon can be replaced by regulations have been set up to protect our sanitary a halogen (chlorine or bromine) under certain con systems from these compounds. It is interesting to ditions; energy must be applied to the reaction: note that some substances we may use on a daily Since the product can continue to react with basis contain water-insoluble chemicals that we con the Br2 in the reaction vessel, a mixture of brominat tinually wash dmvll the drain: hair spray, furniture ed products is the end result. This type of reaction is polish, spot removt'rs, and shampoo used for head termed a free-radical reaction. This is not an addi lice, just to name a few. Nature is ,1bll' 10 contend with tion reaction as seen in the case that follows. small amounts of these substances since microorgan isms degrade these poisons to a certain extent. H H H Another physical property of hydrocarbons is I I I heal or their high volatility. Volatility refers to the ability of H-C-C-C-H + Br2 ) a substance to evaporate, that is, to go from the liq I I I UVlighl uid state to the gaseous state. Highly volatile com H H H pounds can pose problems. If a substance vaporizes H H H easily, the flammability of the substance increases. I I I (For this reason, it is imperative to keep all flames to H-C-C-C-H + HAr a minimum in the organic lab.) Most of the vapors I I I are also dangerous to inhale and some are very II Br H odorous. Strict ventilation procedures must be fol lowed when dealing with hydrocarbons inside and outside the laboratory. Unsaturated hydrocarbons contain double or Since hydrocarbons are part of our everyday triple bonds that are susceptible to attack by many lives, their use should be limited, their disposal chemical reagents, making them much more reac should be monitored, and they should be reused or tive than the saturated hydrocarbons. Double bonds recycled whenever possible. For example, used that are not part of an aromatic ring are subject to motor oil is considered a hazardous waste. Mobil basic addition reactions. The bromination of an and Exxon are two oil companies that have imple alkene is a common addition reaction: mented recycling programs on a national basis. The amount of used oil that can potentially be recycled yearly could produce enough electricity to power 900,000 homes for a year. 1 Furthermore, natural sub stitutions of hydrocarbons are available for the H more hazardous household cleaners. As a last re I sort, leftover paints and paint thinners can be of C-H fered to schools and churches. I Hr

'Hans, Mick. "Hilzilrds thai lurk under the sink." Safellf and Hmllh, May 1992:72. . . (What are the names of the above compounds?)

216 Reactivity of Hydrocarbons In both the bromination reactions, a positive re It should be emphasized that carbons on aro action is apparent by the loss of the brownish-orange matic rings do not behave in the same manner as Br2 color as this molecule adds to the compound. the aliphatic double-bonded compounds. The aro matic rings possess extra stability. Alkyl sub Oxidation stituents on the ring, however, may be reactive. For example, ethylbenzene can be made to brominate at The double bond of the alkenes is susceptible to at the ethyl carbons if exposed to enough energy. tack by oxidizing agents such as KMn041 potassium permanganate. The product is a diol, a compound that contains two -OH groups:

o-CH2-CH ~ 3 UV light CH -CH=CH-CH + KMn04(aq) - 3 3 Br OH OH I I o-~H-CH3 CH3-CH-CH-CH3 + Mn02

Potassium permanganate solution is purple. As with other saturated hydrocarbons, a The indication of a positive test is the loss of this mixture of brominated products may result. Also, purple color and the appearance of brown man note that the benzene ring goes through the reac ganese dioxide, Mn02' tion unchanged.

217 218 Name ___~ ______Date

Laboratory Instructor ______. ______

PRE-LABORATORY ASSIGNMENT Reactivity of Hydrocarbons

1. In what state of matter would you expect decane to be found at room temperature?

2. What does it mean when a substance is described as volatile?

3. Write the chemical equation for the reaction between 2-pentene and bromine. Also, name the product that is formed.

4. What does the term polyunsaturated mean?

5. Draw the structure of the compound that forms when I-hexene is oxidized with KMn04 solution. Name the compound.

6. Draw the structure for toluene.

219 220 PROCEDURE Reactivity of Hydrocarbons

DO NOT INHALE HYDROCARBON FUMES. KEEP ALL OF THE SUBSTANCES UNDER THE HOOD AT ALL TIMES! AVOID SKIN CONTACT.

1. Physical properties of hydrocarbons (Demonstrations)

a. Solubility. Mix 2 mL of hexane with 1 mL of water in a small test tube. Note which substance is on the top and which substance is at the bottom. Shake well and record your observations. Repeat this procedure replacing water with toluene.

b. Volatility. Set 3 mL of hexane under the hood in a small glass dish (crystallization dish) at the beginning of class. Into another glass dish add 3 mL of octane and set it next to the hexane dish. Monitor the amount of time it takes for the liquids to evap orate completely. Record the times on your data sheet. Look up the boiling points of these substances and record them in the data table.

2. Chemical reactivity of hydrocarbons (Test: hexane, cyclohexene, toluene, and your unknown) *Speciai Warning!

Do not get bromine on your skin. It can cause serious chemical burns. Also, avoid vapor contact.

a. Reaction 'with Br2' Put three drops of the liquid to be tested in a small test tube. Add 2-3 drops of the bromine reagent. The disappearance of the bromine color indi cates a positive reaction. If the test is negative, place the test tube under the UV light (Avoid direct eye contact with the UV light) for up to 2 minutes and note if the color disappears.

)THE LIQUID IN THE TEST TUBE SHOULD BE POURED INTO A WASTE JAR LABELED "BROMINATED HYDROCARBON WASTE."

b. Reaction with KMn04' On a white spot plate, place 8 drops of alcohol-free acetone followed by 2 drops of the compound to be tested. Mix thoroughly. Add 2-3 drops of the potassium permanganate solution and again mix thoroughly with a stirring rod. Any change in color should be noted in the first 10 seconds. Brown manganese dioxide formation indicates a positive reaction.

) DISPOSE OF THE REMAINS OF THIS TEST IN A JAR LABELED "PER MANGANATE-HYDROCARBON WASTE."

3. Testing properties of everyday substances (Test: mineral spirits, motor oil, and Vaseline)

221 Exploring Chemistry a. Perform the KMn04 test on each substance as directed in part 2b but instead of using a spot plate, perform the tests with small test tubes. After adding the sub stances to acetone, note the solubility behaviors and record these in your data table along with the results of the test.

>DISPOSE OF THE CONTENTS IN A JAR LABELED "PERMANGANATE HYDROCARBON WASTE."

222 Name ______Date

Laboratory Instructor ______~ ______~

STUDENT REPORT Reactivity of Hydrocarbons

A. Data Sheet

la.

Substances Solubility Observations

Hexane + water

Hexane + toluene

Which substance is more dense, hexane or water? lb. Volatility

Compound Evaporation time Boiling point

Hexane

Octane

Would you expect 2-methylnonane to be ___ more or ____,~.JJ volatile than heptane? 2a.

Addition of Br2: Reaction? Addition of UV light: Reaction? Compound observations observations

Hexane

Cyclohexene

Toluene

Unknown

Write out the chemical equation for each reaction. If the test was positive, show the product that formed. If the test was negative, write NR, indicating that no reaction took place. Finally, name the reactants and products that formed.

223 Exploring Chemistry 2b.

Compounds Addition of KMn0-l: observations Reaction?

Hexane

Cyclohexene

Toluene

Unknown

Write out the chemical equation for each reaction and name the reactants and products. If the test was pos itive, show the product that formed. If the test was negative, write NR, indicating no reaction took place.

Unknown ____ was a saturated, unsaturated compound. (circle one) 3.

Reaction with KMn04: Solubility with Substance observa tions Reaction acetone?

Mineral spirits

Motor oil

Vaseline

The "household products" appear to be ___ saturated, _____ unsaturated compounds (check one).

B. Reflective Exercises

1. Complete the following reactions and name the product that forms. If no reaction occurs, write NR.

224 Reactivity of Hydrocarbons c.

CH b + KMn04 ---.

e. 0 + KMn04 ---.

2. Would you expect hexane to be more soluble in water or in methylene chloride, CH2Cl2? Why?

3. Speculate on the reason why many household hydrocarbons are saturated compounds.

4. When butane is exposed to Br2 and irradiated with UV light, many products are possible. Draw out the structure of six of the many possible products.

225 Explorin~ Chemistry 5. You tested an unknown compound in the laboratory and had the following observations: no re,lC tion with bromine until irradiiltion with UV light; negative test results when mixed with KMn04' Which of the following is/ are a possible molecular formula for this unknown?

b. =CHz

6. Look up the boiling points of hexane and octane. Use your data from the volatility test to explain the relationship between volatility and boiling point.

226