Thus, this molecules can form two stereoisomers: one that has the two chlorine atoms on the same side of the double bond, and the other where the chlorines reside on opposite sides of the double bond.
For this section, we are not concerned with the naming that is also included in this video tutorial. The cis-trans naming system can be used to distinguish simple isomers, where each carbon of the double bond has a set of identical groups attached to it. For example, in Figure 8. The cis and trans system, identifies whether identical groups are on the same side cis of the double bond or if they are on the opposite side trans of the double bond.
For example, if the hydrogen atoms are on the opposite side of the double bond, the bond is said to be in the trans conformation.
When the hydrogen groups are on the same side of the double bond, the bond is said to be in the cis conformation. Notice that you could also say that if both of the chlorine groups are on the opposite side of the double bond, that the molecule is in the trans conformation or if they are on the same side of the double bond, that the molecule is in the cis conformation. To determine whether a molecule is cis or trans , it is helpful to draw a dashed line down the center of the double bond and then circle the identical groups, as shown in figure 8.
Both of the molecules shown in Figure 8. Thus, the cis and trans designation, only defines the stereochemistry around the double bond, it does not change the overall identity of the molecule.
However, cis and trans isomers often have different physical and chemical properties, due to the fixed nature of the bonds in space. Cis-trans isomerism also occurs in cyclic compounds. In ring structures, groups are unable to rotate about any of the ring carbon—carbon bonds.
Therefore, groups can be either on the same side of the ring cis or on opposite sides of the ring trans. For our purposes here, we represent all cycloalkanes as planar structures, and we indicate the positions of the groups, either above or below the plane of the ring.
It relates to our consumption of dietary fats. Inappropriate or excessive consumption of dietary fats has been linked to many health disorders, such as diabetes and atherosclerosis, and coronary heart disease. So what are the differences between saturated and unsaturated fats and what are trans fats and why are they such a health concern? Photo from: TyMaHe. The most common form of dietary fats and the main constituent of body fat in humans and other animals are the triglycerides TAGs.
TAGs, as shown in figure 8. In this section, we will focus on the structure of the long fatty acid tails, which can be composed of alkane or alkene structures. Chapter 10 will focus more on the formation of the ester bonds. Notice that each triglyceride has three long chain fatty acids extending from the glycerol backbone.
Each fatty acid can have different degrees of saturation and unsaturation. Structure adapted from: Wolfgang Schaefer. Fats that are fully saturated will only have fatty acids with long chain alkane tails.
Saturated fats are common in the American diet and are found in red meat, dairy products like milk, cheese and butter, coconut oil, and are found in many baked goods.
Saturated fats are typically solids at room temperature. This is because the long chain alkanes can stack together having more intermolecular London dispersion forces. This gives saturated fats higher melting points and boiling points than the unsaturated fats found in many vegetable oils.
Most of the unsaturated fats found in nature are in the cis -conformation, as shown in Figure 8. Note that the fatty acids shown in Figure 8.
When the fatty acids from the TAG shown in Figure 8. Thus, monounsaturated and polyunsaturated fats cannot stack together as easily and do not have as many intermolecular attractive forces when compared with saturated fats. As a result, they have lower melting points and boiling points and tend to be liquids at room temperature.
It has been shown that the reduction or replacement of saturated fats with mono- and polyunsaturated fats in the diet, helps to reduce levels of the low-density-lipoprotein LDL form of cholesterol, which is a risk factor for coronary heart disease. Trans-fats, on the other hand, contain double bonds that are in the trans conformation.
Thus, the shape of the fatty acids is linear, similar to saturated fats. Trans fats also have similar melting and boiling points when compared with saturated fats. However, unlike saturated fats, trans-fats are not commonly found in nature and have negative health impacts. Trans-fats occur mainly as a by-product in food processing mainly the hydrogenation process to create margarines and shortening or during cooking, especially deep fat frying.
In fact, many fast food establishments use trans fats in their deep fat frying process, as trans fats can be used many times before needing to be replaced.
Consumption of trans fats raise LDL cholesterol levels in the body the bad cholesterol that is associated with coronary heart disease and tend to lower high density lipoprotein HDL cholesterol the good cholesterol within the body. Trans fat consumption increases the risk for heart disease and stroke, and for the development of type II diabetes. The risk has been so highly correlated that many countries have banned the use of trans fats, including Norway, Sweden, Austria and Switzerland.
This measure is estimated to prevent 20, heart attacks and 7, deaths per year. Which compounds can exist as cis-trans geometric isomers? Draw them. All four structures have a double bond and thus meet rule 1 for cis-trans isomerism. This compound meets rule 2; it has two nonidentical groups on each carbon atom H and Cl on one and H and Br on the other.
It exists as both cis and trans isomers:. This compound meets rule 2; it has two nonidentical groups on each carbon atom and exists as both cis and trans isomers:. Which compounds can exist as cis-trans isomers? What are cis-trans geometric isomers?
What two types of compounds can exhibit cis-trans isomerism? Classify each compound as a cis isomer, a trans isomer, or neither. Cis-trans isomers are compounds that have different configurations groups permanently in different places in space because of the presence of a rigid structure in their molecule. Alkenes and cyclic compounds can exhibit cis-trans isomerism.
The situation becomes more complex when there are 4 different groups attached to the carbon atoms involved in the formation of the double bond. The cis-trans naming system cannot be used in this case, because there is no reference to which groups are being described by the nomenclature. For example, in the molecule below, you could say that the chlorine is trans to the bromine group, or you could say the chlorine is cis to the methyl CH 3 group.
Thus, simply writing cis or trans in this case does not clearly delineate the spatial orientation of the groups in relation to the double bond.
Naming the different stereoisomers formed in this situation, requires knowledge of the priority rules. Recall from chapter 5 that in the Cahn-Ingold-Prelog CIP priority system, the groups that are attached to the chiral carbon are given priority based on their atomic number Z. Atoms with higher atomic number more protons are given higher priority i. E comes from the German word entgegen, or opposite. Thus, when the higher priority groups are on the opposite side of the double bond, the bond is said to be in the E conformation.
Z , on the other hand, comes from the German word zusammen, or together. Thus, when the higher priority groups are on the same side of the double bond, the bond is said to be in the Z conformation. As we saw in Chapter 7, small alkanes can be formed by the process of thermal cracking.
This process also produces alkenes and alkynes. In comparison to alkanes, alkenes and alkynes are much more reactive. In fact, alkenes serve as the starting point for the synthesis of many drugs, explosives, paints, plastics and pesticides.
Since combustion reactions were covered heavily in Chapter 7, and combustion reactions with alkenes are not significantly different than combustion reactions with alkanes, this section will focus on the later four reaction types. Most reactions that occur with alkenes are addition reactions.
As the name implies, during an addition reaction a compound is added to the molecule across the double bond. The result is loss of the double bond or alkene structure , and the formation of the alkane structure. The reaction mechanism of a reaction describes how the electrons move between molecules to create the chemical reaction.
Note that in reaction mechanism diagrams, as shown in Figure 8. The reaction mechanism for a generic alkene addition equation using the molecule X-Y is shown below:. Reaction mechanism of a generic addition reaction.
In this reaction, an electron from the carbon-carbon double bond of the alkene attacks an incoming molecule XY causing the breakage of the carbon-carbon double bond lefthand diagram and formation of a new bond between one of the alkene carbons and molecule X.
The original electron from X that was participating in the shared bond with Y, is donated to Y causing the breakage of the X-Y bond. In the intermediate state middle diagram , the alkene is carrying a positively charged carbon ion, called a carbocation , and Y is in a negatively charged anion state. The negative anion is attracted to the positively charged carbocation and donates the two electrons to form the C-Y bond and complete the product of the addition reaction righthand diagram.
Addition reactions convert an alkene into an alkane by adding a molecule across the double bond. There are four major types of addition reactions that can occur with alkenes, they include: Hydogenation, Halogenation, Hydrohalogenation, and Hydration. In a Hydrogenation reaction, hydrogen H 2 is added across the double bond, converting an unsaturated molecule into a saturated molecule. Note that the word hydrogen is found in this reaction name, making it easier to remember and recognize: Hydrogen -ation.
In a hydrogenation reaction, the final product is the saturated alkane. In a Halogenation reaction group 7A elements the halogens are added across the double bond. The most common halogens that are incorporated include chlorine Cl 2 , bromine Br 2 , and Iodine I 2.
Notice that the term halogen is found in this reaction name, making it easier to remember and recognize: Halogen -ation. In halogenation reactions the final product is haloalkane.
In Hydrohalogenation , alkenes react with molecules that contain one hydrogen and one halogen. Hence the name Hydro — Halogen -ation. HCl and HBr are common hydrohalogens seen in this reaction type. In hydrohalogenation, the hydrohalogen is a polar molecule, unlike the nonpolar molecules observed in the halogenation and hydrogenation reactions.
In the case of the hydrohalogen, the end of the molecule containing hydrogen is partially positive, while the end of the molecule containing the halogen is partially negative.
Thus, when the negatively charged electron from the alkene double bond attacks the hydrohalogen, it will preferentially attack the hydrogen side of the molecule, since the electron will be attracted to the partial positive charge. The halogen will then form the negatively charged anion observed in the intermediate structure and attach second during the addition reaction.
The final product is a haloalkane. Just like when your are feeling thirsty, the terms hydration and dehydration refer to water.
Hydration means the addition of water to a molecule, just like when you feel fully hydrated or full of water, while dehydration means the removal or elimination of water, just as when you are feeling dehydrated and need some water to drink. Similar to the hydrohalogenation reaction above, water is also a polar molecule.
In this case, the water is split into two groups to be added across the double bond of the alkene. It is split into the H- and the -OH components. Similar to the hydrohalogenation reaction, the hydrogen adds first, as it carries the partial positive charge.
In more complex molecules, hydrohalogenation and hydration reactions can lead the formation of more than one possible product. For example, if 2-methylpropene [ CH 3 2 CCH 2 ] reacts with water to form the alcohol, two possible products can form, as shown below. However, the addition reaction is not random. One of the products is the major product being produced in higher abundance while the other product is the minor product.
This occurs because the carbocation intermediate that forms as the reaction proceeds is more stable when it is bonded to other carbon atoms, than when it is bonded with hydrogen atoms, as seen in the example below:. In each reaction, the reagent adds across the double bond. Write the equation for each reaction.
What is the principal difference in properties between alkenes and alkanes? How are they alike? If C 12 H 24 reacts with HBr in an addition reaction, what is the molecular formula of the product? Alkenes undergo addition reactions; alkanes do not. Both burn. Complete each equation. In an elimination reaction a molecule loses a functional group, typically a halogen or an alcohol group, and a hydrogen atom from two adjacent carbon atoms to create an alkene structure. Elimination reactions are essentially the reverse reaction of the hydration and hydrohalogenation addition reactions.
Elimination reactions can also occur with the removal of water from alcohol. A rearrangement reaction is a specific organic reaction that causes the alteration of the structure to form an isomer.
With alkene structures, rearrangement reactions often result in the conversion of a cis -isomer into the trans conformation. Due to the high reactivity of alkenes, they usually undergo addition reactions rather than substitutions reactions. The exception is the benzene ring.
The double-bonded structure of the benzene ring gives this molecule a resonance structure such that all of the carbon atoms in the ring share a continually rotating partial bond structure.
Thus, the overall structure is very stable compared to other alkenes and benzene rings do not readily undergo addition reactions. They behave more similarly to alkane structure and lack chemical reactivity. One of the few types of reactions that a benzene ring will undergo is a substitution reaction. Recall from Chapter 7 that in substitution reactions an atom or group of atoms is replaced by another atom or group of atoms.
Halogenation is a common substitution reaction that occurs with benzene ring structures. In the diagram below, notice that the hydgrogen atom is substituted by one of the bromine atoms. A polymer is as different from its monomer as a long strand of spaghetti is from a tiny speck of flour. For example, polyethylene, the familiar waxy material used to make plastic bags, is made from the monomer ethylene—a gas. Because these additions proceed by way of polar or ionic intermediates, the rate of reaction is greater in polar solvents, such as nitromethane and acetonitrile, than in non-polar solvents, such as cyclohexane and carbon tetrachloride.
Only one product is possible from the addition of these strong acids to symmetrical alkenes such as ethene and cyclohexene. However, if the double bond carbon atoms are not structurally equivalent, as in molecules of 1-butene, 2-methylbutene and 1-methylcyclohexene, the reagent conceivably may add in two different ways.
This is shown for 2-methylbutene in the following equation. When addition reactions to such unsymmetrical alkenes are carried out, we find that one of the two possible constitutionally isomeric products is formed preferentially.
Selectivity of this sort is termed regioselectivity. In the above example, 2-chloromethylbutane is nearly the exclusive product. Similarly, 1-butene forms 2-bromobutane as the predominant product on treatment with HBr.
After studying many addition reactions of this kind, the Russian chemist Vladimir Markovnikov noticed a trend in the structure of the favored addition product. In more homelier vernacular this rule may be restated as, " Them that has gits.
It is a helpful exercise to predict the favored product in examples such as those shown below:. Empirical rules like the Markovnikov Rule are useful aids for remembering and predicting experimental results. Indeed, empirical rules are often the first step toward practical mastery of a subject, but they seldom constitute true understanding.
The Markovnikov Rule, for example, suggests there are common and important principles at work in these addition reactions, but it does not tell us what they are. The next step in achieving an understanding of this reaction must be to construct a rational mechanistic model that can be tested by experiment. Since we know that these acids do not react with alkanes, it must be the pi-electrons of the alkene double bond that serve as the base.
This two-step mechanism is illustrated for the reaction of ethene with hydrogen chloride by the following equations. An energy diagram for this two-step addition mechanism is shown to the left. From this diagram we see that the slow or rate-determining step the first step is also the product determining step the anion will necessarily bond to the carbocation site. Electron donating double bond substituents increase the reactivity of an alkene, as evidenced by the increased rate of hydration of 2-methylpropene two alkyl groups compared with 1-butene one alkyl group.
As expected, electron withdrawing substituents, such as fluorine or chlorine, reduce the reactivity of an alkene to addition by acids vinyl chloride is less reactive than ethene. George Hammond formulated a useful principle that relates the nature of a transition state to its location on the reaction path. This Hammond Postulate states that a transition state will be structurally and energetically similar to the species reactant, intermediate or product nearest to it on the reaction path.
In strongly exothermic reactions the transition state will resemble the reactant species. In strongly endothermic conversions, such as that shown to the right, the transition state will resemble the high-energy intermediate or product, and will track the energy of this intermediate if it changes. This change in transition state energy and activation energy as the stability of the intermediate changes may be observed by clicking the higher or lower buttons to the right of the energy diagram.
Three examples may be examined, and the reference curve is changed to gray in the diagrams for higher magenta and lower green energy intermediates.
The carbocation intermediate formed in the first step of the addition reaction now assumes a key role, in that it directly influences the activation energy for this step. Independent research shows that the stability of carbocations varies with the nature of substituents, in a manner similar to that seen for alkyl radicals.
The exceptional stability of allyl and benzyl cations is the result of charge delocalization, and the stabilizing influence of alkyl substituents, although less pronounced, has been interpreted in a similar fashion. From this information, applying the Hammond Postulate, we arrive at a plausible rationalization of Markovnikov's rule. When an unsymmetrically substituted double bond is protonated, we expect the more stable carbocation intermediate to be formed faster than the less stable alternative , because the activation energy of the path to the former is the lower of the two possibilities.
This is illustrated by the following equation for the addition of hydrogen chloride to propene. Note that the initial acid-base equilibrium leads to a pi-complex which immediately reorganizes to a sigma-bonded carbocation intermediate. The following energy diagram summarizes these features. Note that the pi-complex is not shown, since this rapidly and reversibly formed species is common to both possible reaction paths. A more extensive discussion of the factors that influence carbocation stability may be accessed by Clicking Here.
The formation of carbocations is sometimes accompanied by a structural rearrangement. Such rearrangements take place by a shift of a neighboring alkyl group or hydrogen, and are favored when the rearranged carbocation is more stable than the initial cation. The addition of HCl to 3,3-dimethylbutene, for example, leads to an unexpected product, 2-chloro-2,3-dimethylbutane, in somewhat greater yield than 3-chloro-2,2-dimethylbutane, the expected Markovnikov product.
To see this rearrangement click the " Show Mechanism " button to the right of the equation. Another factor that may induce rearrangement of carbocation intermediates is strain. However, the rearrangement also expands a strained four-membered ring to a much less-strained five-membered ring, and this relief of strain provides a driving force for the rearrangement.
A three-dimensional projection view of the rearrangement may be seen by clicking the " Other View " button. The atom numbers colored red for the pinene structure are retained throughout the rearrangement to help orient the viewer.
The green numbers in the final product represent the proper numbering of this bicyclic ring system. The propensity for structural rearrangement shown by certain molecular constitutions, as illustrated above, serves as a useful probe for the intermediacy of carbocations in a reaction.
We shall use this test later. Please do not block ads on this website. Alkenes can be prepared by elimination reactions in which a small molecule such as water is eliminated from a reactant molecule such as an alkanol or haloalkane alkyl halide.
Two elimination reactions commonly used to synthesise alkenes are. In order to eliminate water, a solution of potassium hydroxide in alcohol is used referred to as alcoholic KOH.
If aqueous KOH were used, the result would be a substitution reaction producing an alcohol and a salt! Play the game now! The physical properties of alkenes such as boiling point and solubility are related to the non-polar nature of alkene molecules. Alkenes are non-polar molecules. Only weak intermolecular forces dispersion or London forces act between the molecules. Since little energy is required to disrupt these weak intermolecular forces, alkenes are expected to have low melting and boiling points.
The data in the table below compares the number of carbon atoms in an alkene chain with its boiling point:.
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