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Properties of Alkenes http://homepage.mac.com/stray/ib/chem/SSC/topic11.html

PROPERTIES OF ALKANES  1. Define what is meant by "homologous series". Describe their features in terms of physical and chemical properties.  2. PREDICT and EXPLAIN the trends in boiling points of members of a homologous series (from C1-C6)  3. Define what is meant by "isomer". Give an example of a set of isomers.  4. PREDICT and EXPLAIN the trend in boiling points of isomers. Using the examples from #3, arrange the boiling points in increasing/ decreasing order.

 REACTIONS OF ALKANES  Introduction: Comment on the reactivity of alkanes. (high or low?) Explain your answer.

 Specific reactions:  For this part, make sure to highlight the chemical equations (focusing on a sample reactant, the products they will produce, and catalyst, if present)  1. COMBUSTION (complete and incomplete)  2. REACTION OF ALKANES WITH CHLORINE AND BROMINE  Added notes for #2: This reaction involves the reactants undergoing a special mechanism called the free-radical mechanism. Identify the three stages of this mechanism and the equations that represent these stages. What is homolytic fission and where is this seen in the mechanism? I suggest looking for videos to better illustrate the mechanism. Embed this video in the wikispace using the same format as you've been using in wordpress.

A homologous series is a set of compounds whose components differ by a single repeating functional group...in the case of (straight chain) alkanes, CH2...and their general formula is CnH2n+2.
 * Homologous Series**

 Homologous Series: is a series of compounds with the SAME GENERAL FORMULA, SIMILAR CHEMICAL PROPERTIES, and a GRADUATION IN PHYSICAL PROPERTIES, where each member differs from the previous member by a CH2 group.

For example: ALKANES: General formula is C(n)H(2n+2) eg. C2H6 or C3H8 or C4H10. All of these are saturated molecules, so they contain the maximum number of hydrogen atoms per molecule, ie. all bonds are single and there are no unused electron pairs. This means that these will not be able to undergo addition reactions because there are no free electron pairs to bond to. Therefore, alkanes have SIMILAR CHEMICAL PROPERTIES. Also, no alkane is polar. Alkanes have a GRADUATION IN PHYSICAL PROPERTIES because as there are more CH2 groups, the molecule's mass and size increases, hence the strength of the acting intermolecular forces, the Van der Waal's forces is increasing therefore the bigger the molecule, the stronger the intermolecular forces, therefore the higher the boiling point.   http://wiki.answers.com/Q/What_are_the_physical_and_chemical_properties_of_homologous_series


 * The Trends in Boiling Point **

As we can see from the graph, as the chain length of Carbon atoms gets longer, the boiling point is higher.

The boiling points of alkanes increase as the chains get longer (increased number of electrons -> increased van de Waal's forces), increasing rapidly initially but flattening off (since the number of additional CH2 units required to double the chain length increases rapidly...so it flattens of...or you could just believe it)

isomer |ˈīsəmər| noun  1 Chemistry each of two or more compounds with the same formula but a different arrangement of atoms in the molecule and different properties.

taken from - mac dictionary  <span style="font-family: arial,sans-serif; font-size: medium; font-weight: normal; line-height: 19px;">**Pentane boiling point — 36 °C** 2-methlybutane boiling point - 27.7 °C (300.9 K) 2,2-dimethlypropane boiling point - 9.5 °C (283 K)

<span style="font-family: Verdana,sans-serif; font-size: 12px; line-height: normal;">Alkanes are not very reactive because of the stability of their bonds. The carbon hydrogen bonds found in alkanes are virtually nonpolar. Also, carbon and hydrogen have no lone pairs of electrons. This means that they are not subject to attack by nucleophiles or electrophiles.

<span style="font-family: Verdana,sans-serif; font-size: 12px; line-height: normal; padding: 0px;"> Read more: <span style="color: #003399; outline-color: initial; outline-style: none; outline-width: initial; padding: 0px; text-decoration: none;">[]

<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 16pt; line-height: 19px;">1. Combustion
<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">The combustion of carbon compounds, especially hydrocarbons, has been the most important source of heat energy for human civilizations throughout recorded history. The practical importance of this reaction cannot be denied, but the massive and uncontrolled chemical changes that take place in combustion make it difficult to deduce mechanistic paths. Using the combustion of propane as an example, we see from the following equation that every covalent bond in the reactants has been broken and an entirely new set of covalent bonds have formed in the products. No other common reaction involves such a profound and pervasive change, and the mechanism of combustion is so complex that chemists are just beginning to explore and understand some of its elementary features. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">CH 3 -CH 2 -CH 3 + 5 O 2 ——** > ** 3 CO 2 + 4 H 2 O + heat <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">__Two points concerning this reaction are important__:

<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 1. **Since all the covalent bonds in the reactant molecules are broken, the quantity of heat evolved in this reaction is related to the strength of these bonds (and, of course, the strength of the bonds formed in the products). Precise heats of combustion measurements can provide useful iinformation about the structure of molecules. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 2. **The stoichiometry of the reactants is important. If insufficient oxygen is supplied some of the products will consist of carbon monoxide, a highly toxic gas. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">CH 3 -CH 2 -CH 3 + 4 O 2 ——** > ** CO 2 + 2 CO + 4 H 2 O + heat

<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 15px; line-height: 19px;">

<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 16pt; line-height: 19px;">2. Halogenation
<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">Halogenation is the replacement of one or more hydrogen atoms in an organic compound by a halogen (fluorine, chlorine, bromine or iodine). Unlike the complex transformations of combustion, the halogenation of an alkane appears to be a simple ** substitution reaction ** in which a C-H bond is broken and a new C-X bond is formed. The chlorination of methane, shown below, provides a simple example of this reaction.

<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> CH 4 + Cl 2 + energy ——** > ** CH 3 Cl + HCl

<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">Since only two covalent bonds are broken (C-H & Cl-Cl) and two covalent bonds are formed (C-Cl & H-Cl), this reaction seems to be an ideal case for mechanistic investigation and speculation. However, one complication is that all the hydrogen atoms of an alkane may undergo substitution, resulting in a mixture of products, as shown in the following __unbalanced equation__. The relative amounts of the various products depend on the proportion of the two reactants used. In the case of methane, a large excess of the hydrocarbon favors formation of methyl chloride as the chief product; whereas, an excess of chlorine favors formation of chloroform and carbon tetrachloride. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">CH 4 + Cl 2 + energy ——** > ** CH 3 Cl + CH 2 Cl 2 + CHCl 3 + CCl 4 + HCl <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">The following facts must be accomodated by any reasonable mechanism for the halogenation reaction.

<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 1. **The reactivity of the halogens decreases in the following order: F 2 > Cl 2 > Br 2 > I 2. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 2. **We shall confine our attention to chlorine and bromine, since fluorine is so explosively reactive it is difficult to control, and iodine is generally unreactive. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 3. **Chlorinations and brominations are normally exothermic. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 4. **Energy input in the form of heat or light is necessary to initiate these halogenations. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 5. **If light is used to initiate halogenation, thousands of molecules react for each photon of light absorbed. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 6. **Halogenation reactions may be conducted in either the gaseous or liquid phase. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 7. **In gas phase chlorinations the presence of oxygen (a radical trap) inhibits the reaction. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> ** 8. **In liquid phase halogenations radical initiators such as peroxides facilitate the reaction.

<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">The most plausible mechanism for halogenation is a chain reaction involving neutral intermediates such as free radicals or atoms. The weakest covalent bond in the reactants is the halogen-halogen bond (Cl-Cl = 58 kcal/mole; Br-Br = 46 kcal/mole) so the initiating step is the homolytic cleavage of this bond by heat or light, note that chlorine and bromine both absorb visible light (they are colored). A chain reaction mechanism for the chlorination of methane [|has been described]. <span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;"> Bromination of alkanes occurs by a similar mechanism, but is slower and more selective because a bromine atom is a less reactive hydrogen abstraction agent than a chlorine atom, as reflected by the higher bond energy of H-Cl than H-Br.

<span style="font-family: Arial,Helvetica,Verdana,sans-serif; font-size: 11pt; line-height: 1.3;">http://www2.chemistry.msu.edu:80/faculty/reusch/VirtTxtJml/funcrx1.htm

<span style="color: #000000; font-family: Helvetica,Arial; font-size: medium; line-height: normal;">**Substitution reactions**

These are reactions in which one atom in a molecule is replaced by another atom or group of atoms. Free radical substitution for A' level purposes involves breaking a carbon-hydrogen bond in alkanes such as


 * <span style="color: #000000; font-family: Helvetica,Arial;">methane || <span style="color: #000000; font-family: Helvetica,Arial;">CH4 ||
 * <span style="color: #000000; font-family: Helvetica,Arial;">ethane || <span style="color: #000000; font-family: Helvetica,Arial;">CH3CH3 ||
 * <span style="color: #000000; font-family: Helvetica,Arial;">propane || <span style="color: #000000; font-family: Helvetica,Arial;">CH3CH2CH3 ||

A new bond is then formed to something else. It also happens in alkyl groups like methyl, ethyl (and so on) wherever these appear in more complicated molecules.


 * <span style="color: #000000; font-family: Helvetica,Arial;">methyl || <span style="color: #000000; font-family: Helvetica,Arial;">CH3 ||
 * <span style="color: #000000; font-family: Helvetica,Arial;">ethyl || <span style="color: #000000; font-family: Helvetica,Arial;">CH3CH2 ||

For example, ethanoic acid is CH3COOH and contains a methyl group. The carbon-hydrogen bonds in the methyl group behave just like those in methane, and can be broken and replaced by something else in the same way. A simple example of substitution is the reaction between methane and chlorine in the presence of UV light (or sunlight).


 * || <span style="color: #000000; font-family: Helvetica,Arial;">CH4 + Cl2[[image:http://www.chemguide.co.uk/mechanisms/freerad/arrow.GIF width="68" height="16" align="absmiddle"]]CH3Cl + HCl ||

<span style="color: #000000; font-family: Helvetica,Arial;"> Notice that one of the hydrogen atoms in the methane has been replaced by a chlorine atom. That's substitution. <span style="color: #000000; font-family: Helvetica,Arial; font-size: small;"> Free radicals are atoms or groups of atoms which have a single unpaired electron. A free radical substitution reaction is one involving these radicals. Free radicals are formed if a bond splits evenly - each atom getting one of the two electrons. The name given to this is**//homolytic fission//**.
 * Free radical reactions**

<span style="color: #000000; font-family: Arial,Helvetica;"> To show that a species (either an atom or a group of atoms) is a free radical, the symbol is written with a dot attached to show the unpaired electron. For example:

|| || http://www.chemguide.co.uk/mechanisms/freerad/whatis.html#top = Alkenes = <span style="font-family: Verdana,sans-serif; font-size: 14px; line-height: 21px;"> Alkenes contains a carbon-carbon double bond. This carbon-carbon double bond changes the physicals properties of alkenes. At room temperatue, alkenes exist in all three phases, solid, liquids, and gases. Melting and boiling points of alkenes are similar to that of alkanes, however, isomers of cis alkenes have lower melting points than that of trans isomers. Alkenes display a weak dipole-dipole interactions due to the electron-attracting sp2 carbon. = Alkanes are saturated while alkenes are unsaturated. Reactions: **Hydrogenation--Addition of Hydrogen** When an alkene is hydrogenated, it becomes and alkane. It requires a catalyst--powedered platinum, for example-- and often high heat and pressure though. The following is the hydrogenation of 2-butene: <span style="background-color: #eeeeee; color: #160909; font: 13.0px Courier; line-height: 19.0px; margin: 0.0px 0.0px 0.0px 0.0px;">CH 3 CH=CHCH 3 + H-H ==> CH 3 CH-CHCH 3 or CH 3 CH 2 CH 2 CH 3 | | H H 2-butene butane http://library.thinkquest.org/3659/orgchem/alkenes-alkynes.html **Combustion of Alkenes** The alkenes are highly flammable and burn readily in air, forming carbon dioxide and water,. For example, ethene burns as follows : <span style="background-color: #eeeeee; font: 12.0px Times; line-height: 19.0px; margin: 0.0px 0.0px 0.0px 0.0px;"> **C2H4 + 3 O2 ==> 2 CO2 + 2 H2O** http://www.ucc.ie/academic/chem/dolchem/html/dict/alkenes.html **Polymerisation Reactions due to the Double Bond** When ethylene is heated under great pressure in the presence of a catalyst a large number of the molecules combine to form polythene, (C2H4)n, (i.e. Polyethylene). This particular kind of reaction is called an addition polymerisation and the mechanism by which it takes place is a reaction is a free radical chain reaction. The overall reaction is <span style="background-color: #eeeeee; font: 13.0px Courier; line-height: 19.0px; margin: 0.0px 0.0px 0.0px 0.0px;"> **n(C2H4) ==> (C2H4)n** **Ethene Polyethene** http://www.ucc.ie/academic/chem/dolchem/html/dict/alkenes.html **Halogenation** Halogenation is the addition of halogen atoms to a π-bond system. For example, the addition of bromine to ethene produces the substituted alkane 1,2-dibromoethane. C2H4 (g) + Br2 (tetrachloromethane) ---> CH2Br-CH2Br (aq) <span style="color: #000000; font: normal normal normal 12px/normal Verdana; margin: 0px;">[] <span style="font: normal normal normal 12px/normal Arial; line-height: 15px; margin: 0px; min-height: 14px;">__[]__ Halogens readily add across the double bond of the alkenes to form dihalides code **H2C=CH2 + Cl2 ==> CH2Cl CH2Cl Ethene DiChloroEthane** code **H2C=CH2 + Br2 ==> H2Br CH2Br** **Ethene DiBromoEthane** The decolourisation of bromine is a second test for an unsaturated organic compound. **Addition of Water**Water can add across the double bond of the alkenes to form aliphatic alcohols. This is hydration reaction is catalysed under a number of different conditions. When ethylene and steam are heated (i.e. at 300o Centigrade) under high pressure (i.e. at 70 atm.) in the presence of a catalyst (i.e. phosphoric acid,, on a silica support), ethanol is formed. code **H2C=CH2 + H2O ==> C2H5OH** code code <span style="background-color: #eeeeee; border: 1px solid #cccccc; font-size: 13px; overflow-x: auto; padding: 10px;">H2C=CH2 + HCl ==> C2H5Cl (chloroethane) code code <span style="background-color: #eeeeee; border: 1px solid #cccccc; font-size: 13px; overflow-x: auto; padding: 10px;">H2C=CH2 + H2 ==(catalyst Ni at 180 C)==> ethane code Organic Derivatives =
 * <span style="color: #000000; font-family: Helvetica,Arial;">a chlorine radical || <span style="color: #000000; font-family: Helvetica,Arial;">Cl
 * <span style="color: #000000; font-family: Helvetica,Arial;">a methyl radical || <span style="color: #000000; font-family: Helvetica,Arial;">CH3

Alcohols An alcohol is any compound with an OH group (alcohol group) attached to single bonded hydrocarbons (alkanes). General formula: R-OH (OH - hydroxyl FG)

The four most common alcohols are: code CH3OH

code

methanol (methyl alcohol)
|| code CH3CH2OH

code

ethanol (ethyl alcohol)
|| code CH3CH2CH2OH

code

1-propanol (propyl alcohol)
|| code OH  | CH3CHCH3

code

2-propanol (isopropyl alcohol)
|| Ethanol, when fermented from sugar, is the alcohol in beverages. It can also be made from ethene by the addition of water for nonbeverage use, like an additive to gassoline to make "gasahol." 2-Propanol (better known as isopropyl alcohol) is in (with some water) rubbing alcohol. It is also used in gasoline to prevent freezing of the gas line in automobiles by keeping excess moisture dissolved in the gasoline.

The parent chain of the alcohol must be the longest that includes the carbon holding the OH group. Give the -OH group the lower location number on the chain regardless of where alkyl substituents occur. Name the alkane attached to the OH group and replace the -e with an -ol. For example: code H                   | H H H-C-H H             | |   |   | H-C-C---C---C-H | |  |   |              H O   H   H                | H         Parent chain: butyl -OH group location: 2 Substituents locations: 3-methyl Alkane name: 3-methylbutane Alcohol name: 3-methyl-2-butanol code http://library.thinkquest.org/3659/orgchem/alcohol-ethers.html Functional Group <span style="font-family: Verdana,Arial,Helvetica,sans-serif; font-size: 12px; line-height: 17px;">An atom or group of atoms, such as a carboxyl group, that replaces hydrogen in an organic compound and that defines the structure of a family of compounds and determines the properties of the family.

Aldehydes General formula: R-CHO (C=O - carbonyl FG) Solubility in water: YES Relative boiling point: MODERATE To name, the -e at the end is replaced by -al. Examples: HCHO is methanal

Ketones General formula: R-COR' (R' represents either the same alkyl group as R or a different one) (C=O - carbonyl FG) Solubility in water: YES Naming: The -e at the end of the longest chain alkane is replaced by -one. Relative boiling point: MODERATE Examples: CH3 COCH3 is propanone CH3 COC2 H5 is butan-2-one

Carboxylic Acids General formula: R-COOH (COOH carboxyl FG) Solubility in water: YES Naming: the -e at the end of the longest chain alkane is replaced by -oic acid. Relative boiling point: HIGH Examples: HCOOH is methanoic acid CH3 COOH is ethanoic acid

Halogenoalkanes (alkyl halides) General formula: R-X (X is either F, Cl, Br or I) (halogen FG) Solubility in water: NO Naming: either fluoro-, chloro-, bromo- or iodo- is placed in front of the alkane. Relative boiling point: LOW Examples: CH3 I is iodomethane CH3 CHClCH3 is 2-chloropropane

Amines General formula: R-NH2 (amino FG) Solubility in water: MOST Naming: either add amino- to the front or -amine to the end. Relative boiling point: DEPENDS Examples:

| H - C - N - H  | | H H || (or methylamine) ||
 * CH3-NH2H
 * aminomethane

| | | H - C - C - C - H  | | | H N H / \ H H || (or isopropylamine) ||
 * H H H
 * 2-aminopropane

| | | | | H - C - C - N - C - C - C - H  | | | | | | H H H H H H || - from the Chem book
 * H H H H H
 * ethylpropylamine ||