Organic reactions and mechanisms report 1

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Organic Reactions & Mechanisms Lab Report 1

Experiment 2

Chloe Condon, X00147696, DNA & Forensic Analysis

3rd October 2019

Date of Practical:

3rd October 2019

Title of Practical

Preparation of 4-vinylbenzoic acid by a Wittig reaction in an aqueous medium.

Lab partners:

Meagan Dooley, Emma Egan

Introduction:

The Wittig reaction involves the reaction of an aldehyde or ketone with an ylide that is generated from a phosphonium salt, in order to produce an alkene [1]. An ylide is a neutral molecule that has positive and negative charges on adjacent atoms and are connected by a sigma bond[2].

The Wittig reaction is an important reaction in organic chemistry as it is a useful way of converting aldehydes and ketones into alkenes. An article that mentions a study done by Jean Pierre Schmit, Marcel Piraux and Jean Francois Pilette in Belgium, stated that the Wittig reaction had been used in order to synthesise steroidal side chains as a secondary reaction[3]. This is an example of how the Wittig reaction can be quite useful.

In this practical, 4-bromomethylbenzoic acid underwent the Wittig reaction along with triphenyl benzol, a phosphonium salt and an ylide in order to produce the alkene 4-vinylbenzoic acid.

Reflux condensing is a technique used in this practical in order to dissolve solids in a solution that may usually be difficult to dissolve on their own. Reflux condensers also help prevent the mixture from evaporating and boiling dry. A condenser was needed in this practical to help dissolve the 4-bromobenzoic acid and triphenyl benzol in acetone, and also to help prevent the acetone from evaporating as it is highly volatile.

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The product was analysed using NMR and IR, and the melting point was determined.

NMR is a technique used to determine the position and number of hydrogens in a compound based on the number of signals, their intensity, their splitting pattern and the chemical shift. Only the number of hydrogens is shown on the spectrum. The more groups that are beside the hydrogen, the more it gets shifted down the spectrum. Double and triple bonds can de-shield hydrogens and benzene rings de-shield hydrogens even more than these bonds.

IR spectroscopy is an analytical technique that helps with the identification of the structures of organic molecules. Each peak on the spectrum represents a particular bond in molecules

Method:

center137541000Figure 1 showing reaction scheme for Wittig reaction

Preparation of the phosphonium salt involved the following steps:

2.15g of 4-bromomethylbenzoic acid and 2.62g of triphenylphosphine were dissolved in 30ml of acetone in a 100ml round bottomed flask (RBF)

A small amount of boiling stones was added to the flask and reflux apparatus was set up.

The mixture was refluxed for approximately 35 minutes.

When 35 minutes had passed, the reaction mixture was cooled and the phosphonium salt that was formed was filtered using suction filtration through a Buchner funnel.

Diethyl ether was used in two portions of 10ml to wash the solid through the Buchner funnel.

The solid was allowed t dry under suction for 5 minutes.

Preparation of the 4-vinylbenzoic acid involved the following steps:

The recovered phosphonium salt was transferred to a 250ml RBF along with a magnetic stirrer and was clamped over a magnetic stirring plate.

32ml of aqueous formaldehyde was added to the flask and was vigorously stirred.

A pressure equalised dropping funnel was set up that contained a solution of 2.5g of NaOH that was dissolved in 15ml of water.

This solution was added in small portions to the RBF over a space of 10 minutes.

The mixture was left to stir for an extra 30 minutes before the precipitate was filtered by suction again and washed with two 10ml portions of water.

The filtrate and washings in the flask were acidified to a pH of less than 3 by means of addition of concentrated HCl.

Litmus paper was used in order to test the acidity of the solution.

The precipitate was filtered using suction again. 10ml of water was used to wash the product.

The product was recrystallised using a small amount of hot ethanol. Water was added dropwise to the flask until the solution went cloudy.

The solution was allowed to cool at 0oC for 10 minutes.

Suction filtration was used one last time to recover the product, and the product was dried in a vacuum oven at 40oC for approximately 20 minutes.

The product was analysed using IR, NMR and the melting point was determined.

Results:

Yield Calculation:

Reaction is a 1:1 ratio, so moles are all = 1

Theoretical yield= mass of limiting reagentmolecular weight of limiting reagent x 11 x molecular weight of product12.15g215.1g /mol x 1 mol of A1 mol of B x 148.15g /mol1 mol of product=1.26gTheoretical yield = 1.26g

Actual yield = 2.55g

% yield= Actual yieldTheoretical yield x 1002.55g1.26g x 100=202.38%Figure 2 showing NMR for 4-vinylbenzoic acid (final product)

20478757277100Hb

00Hb

43148255280660Hd00Hd37623755090160156210054806852200275555688511430005185410Ha

00Ha

-29337004728210Hc0Hc25146004947285right57150000center5791200

Figure 3 showing table for NMR analysis of 4-vinybenzoic acid

Chemical shift (ppm) Integration

(no. of Hydrogens) Multiplicity

(no. of adjacent hydrogens + 1) Peak assignment

125.96 1H Triplet CH (Hb)

130.33 2H Doublet CH2 (Ha)

135.98 4H Triplet Aromatic hydrogens (Hc)

142.84 1H Singlet OH (Hd)

-692151190625Figure 4 showing IR of starting material (4-bromomethyl benzoic acid)

Figure 5 showing table of IR analysis of starting material

Frequency (cm-1) Appearance Assignment Functional group Range

(cm-1)

538.86 Tall, sharp C-C bonds C-C 1500-500

1284.26 Tall, sharp C-O stretch C-O 1200-1000

1424.70 Medium, sharp Benzene Aromatic ring 1500-1430

1680.27 Tall, sharp C=O stretch Carbonyl 1800-1600

2972.11 to 2546.13 Short, broad peaks O-H stretch Carboxylic acid 3300-2500

right282575Figure 6 showing IR of final product (4-vinylbenzoic acid)

Figure 7 showing table of IR analysis of final product

Frequency

(cm-1) Appearance Assignment Functional group Range

(cm-1)

536.74 Tall, sharp C-C bond C-C 1500-500

1117.06 Medium, sharp C-O stretch C-O 1200-1000

1258.51 Medium, broad C-O stretch C-O 1200-1000

1674.94 Medium, broad C=O stretch Carbonyl 1800-1600

2822.19 to 2494.97 Short, broad peaks O-H stretch Carboxylic acid 3300-2500

Melting point of product obtained: 138oC

Literature value for melting point of 4-vinylbenzoic acid: 142oC – 144oC [4]

Discussion:

center172402500Figure 8 showing reaction mechanism for the Wittig reaction

The yield that was obtained for the product was 202%. This is a very high, abnormal yield to obtain. It was observed when removing the product from the oven and attempting to carry out determination of the melting point, that the product was still wet. This is a possible reason as to why there was a very high yield, as not all the solvent that remained in the product after vacuum filtration had been evaporated. In future experiments, this problem can be avoided by ensuring the product is kept in the oven for an adequate amount of time, so that all the excess solvent has been evaporated.

The NMR of the product showed that the CH seen in position Hb on the structure above figure 3 is the very first peak observed on the spectrum, and is not pushed downfield by the CH2 it is attached to. It has 2 adjacent hydrogens so it shows as a triplet. CH2 at position Ha in the structure only has 1 adjacent hydrogen so it shows as a doublet. The aromatic hydrogens on the benzene ring are seen around 135.98ppm, much further downfield than the CH and CH2. Finally, the OH group is the last peak to be observed and is the furthest downfield because of its position right next to the C=O group.

The IR of the starting product showed all expected peaks for the structure of 4-bromomethylbenzoic acid. The final product shows similar peaks to the starting product but some were slightly different and off. This may have been due to the product still being wet and containing contaminants that affect the IR comparison to the starting product.

References:

TUD image available at:

[1] Organic-chemistry.org. (2019). Wittig Reaction. [online]

Available at: [Accessed 20 Oct. 2019].

[2] Chem.ucalgary.ca. (2019). Ch17: Wittig reaction. [online]

Available at: [Accessed 20 Oct. 2019].

[3] Pubs.acs.org. (2019). Application of the Wittig reaction to the synthesis of steroidal side chains. Possibility of 3.beta.-phenoxy formation as a secondary reaction. [online]

Available at:

[Accessed 20 Oct. 2019].

[4]

[Accessed 20 Oct. 2019]