Alkenes | Organic Chemistry 2
Nomenclature of Alkenes
Name: the -ane ending of the corresponding alkane is replaced by -ene
1) Find the longest chain that includes the C=C bond
2) Indicate the position of the C=C bond, starting the numbering with the end closest to C=C
3) Add the name of the substituents and their positions (with the smallest number possible) as prefixes
4) Identify the type of stereoisomers (cis-trans for disubstituted alkenes or E,Z for alkenes with 3 or 4 substituents) and add as a prefix
5) Prioritize the -OH functional group over C=C: alkenols
Pi Bond
C=C bond of alkenes:
1 σ bond ⇒ formed from 2 sp2 orbitals of carbon atoms
1 π bond ⇒ formed from 2 p orbitals of carbon atoms
The π bond is weaker (higher energy) and therefore more reactive than the σ bond
C=C bonds are shorter than C-C bonds
Geometry:
The carbons of the C=C bond of an alkene are sp2 hybridized ⇒ trigonal geometry
The angles H-C-H and H-C-C are ~ 120°. H-C-H angles are slightly smaller than H-C-C angles due to the repulsion of hydrogen atoms by electrons in the π bond
Reactivity of alkenes is 'all about the π bond' (mainly: addition reactions)
Properties of Alkenes
Boiling and melting points: similar to corresponding alkanes
Acidity: extremely weak base (pKa = 40)
NMR:
1H δ ~ 5-7 ppm; 13C δ ~ 100-160 ppm
Each H of the C=C bond has different chemical shift and coupling constants.They can be cis, trans or geminal
cis coupling constant: between 6-14Hz
trans coupling constant: between 11-18Hz
geminal coupling constant: between 0-3Hz
E and Z isomers:
The rigidity of the π bond prevents substituent interconversion in alkenes by rotation. This leads to the existence of geometric isomer forms known as E and Z isomers. According to the Cahn-Ingold-Prelog rules, if the two highest priority groups are on the same side, it is called the ‘Z’ isomer (from ‘Zusammen’, meaning ‘together’ in German). Conversely, if they are on opposite sides, it is called the ‘E’ isomer (from 'Entgegen', meaning 'opposite')
Preparation of Alkenes
Alkyl halides dehydrohalogenation (via an E2 mechanism):
- Zaitsev rule: non-bulky bases favor the formation of the most substituted alkene, which is the thermodynamic product. This product is the most stable alkene.
- Hofmann rule: bulky bases favor the formation of the less substituted alkene, which is the kinetic product. This product is often formed more quickly in the reaction.
Alkyl tosylates can also be used as starting materials under similar reactions conditions.
Alcohol dehydration:
Alcohol reactivity: primary < secondary < tertiary
Major product: thermodynamic product (Zaitsev rule)
Degree of Unsaturation
Unsaturated molecules:
Molecules containing a double bond (1 unsaturation), a triple bond (2 unsaturations), and/or a ring (1 unsaturation)
Degree of unsaturation:
Calculation that determines the total number of rings and π bonds in a molecule. It is a valuable tool in organic chemistry for quickly assessing the molecular structure and aiding in the identification of compounds. The formula for calculating the degree of unsaturation is given by:
where:
nC = number of carbons
nN = number of nitrogens
nH = number of hydrogens
nX = number of halogens (F, Cl, Br, I)
Check your knowledge about this Chapter
- Identify the longest carbon chain containing the double bond and use that as the base name.
- Number this chain starting from the end closest to the double bond, making sure that the double bond has the lowest number possible. The position of the double bond is indicated by a number before the -ene end. If there are multiple double bonds, use suffixes such as "diene" or "triene".
- Name and number substituent groups according to their position on the carbon chain, with their positions indicated by numbers separated by commas.
- Alkenes take precedence over alkanes in naming, so if an alkene group is present, the -ene suffix is used instead of -ane.
A π bond is a type of covalent bond formed by the lateral overlap of adjacent p orbitals above and below the bond axis. Unlike σ bonds, which are formed by the head-on overlap of orbitals and allow for free rotation of the bonded atoms, π bonds restrict this rotation due to the parallel orientation of their orbitals. In addition, the π bonds are weaker (higher energy) and therefore more reactive than the σ bonds due to the greater spatial distribution of electron density and weaker overlap in side-to side p orbital interactions.
The geometry of a double bond in alkenes is determined by the arrangement of atoms around the carbon-carbon double bond. The bond angle around each carbon is approximately 120 degrees in a trigonal planar arrangement, leading to a flat structure.
Alkenes have similar physical properties to alkanes, but they tend to have slightly higher boiling points due to the presence of the double bond, which increases the electron density and the van der Waals forces between molecules. However, like alkanes, they are nonpolar and therefore insoluble in water. Their low polarity also causes alkenes to have lower melting and boiling points than polar compounds such as alcohols or ketones of comparable molecular weight.
'Z' stands for 'zusammen,' meaning 'together', while 'E' stands for 'entgegen,' meaning 'opposite'. They indicate whether the higher-priority substituents on a double bond are on the same side ('Z') or opposite sides ('E') based on the Cahn-Ingold-Prelog priority system.
Cis-trans isomerism, also known as geometric isomerism, occurs in alkenes due to the inflexibility of the double bond (there is no interconversion of the substituents of an alkene by rotation). In a cis isomer, the substituents attached to the double-bonded carbons are on the same side, while in a trans isomer, they are on opposite sides.
This form of isomerism is important because cis and trans isomers often have different physical and chemical properties, which lead to different reactivities and interactions in biological systems. For example, a cis alkene is more polar than a trans alkene, giving it a slightly higher boiling point and making it more soluble in polar solvents.
Common methods for preparing alkenes include alcohol dehydration, elimination reactions of alkyl halides, and catalytic hydrogenation of alkynes (see the chapter on alkynes).
Dehydration of alcohols involves the elimination of a water molecule (H2O) from the alcohol to form an alkene. This reaction usually requires an acid catalyst, such as sulfuric acid or phosphoric acid, to proceed. Here are the different steps of the mechanism:
- Protonation of the alcohol by the acid catalyst to form an oxonium ion
- Loss of a water molecule to form a carbocation intermediate
- Elimination of a proton (H+) from an adjacent carbon, facilitated by the conjugate base of the acid catalyst (e.g.,HSO4− or H2PO4−), resulting in the formation of the π bond
Stereoselectivity is crucial in the preparation of alkenes because it determines the geometric isomer that will be formed when a new double bond is created. Alkenes are capable of exhibiting cis-trans (or E-Z) isomerism due to the restricted rotation around the double bond. The stereoselectivity of a reaction can be influenced by the choice of reactants, catalysts, and reaction conditions. For instance, in elimination reactions, such as E2 or E1, the stereochemistry of the starting material and the mechanism will dictate whether a Z (cis) or an E (trans) alkene will be preferred. Understanding and controlling stereoselectivity is vital for synthesizing alkenes with specific configurations, which can have significant implications in fields like pharmaceuticals where the 3D shape of a molecule affects its biological activity.
The degree of unsaturation in a hydrocarbon reflects the number of π bonds and rings present in the compound. One degree of unsaturation is equivalent to either one π bond or one ring.
To calculate it, you can use the formula: (2nC + 2 + nN - nH - nX) / 2, where nC is the number of carbons, nN is the number of nitrogens, nH is the number of hydrogens, and nX is the number of halogens in the molecule.