Storage and / or transportation of ethylenically unsaturated compounds

Abstract

The present invention relates to a method for storing and / or transporting ethylenically unsaturated compounds, which are protected from undesired free radical polymerization using phenol.

Description

[Technical field]

[0001] The present invention relates to a method for storing and / or transporting ethylenically unsaturated compounds, which are protected from undesired free radical polymerization using phenol. [Background technology]

[0002] Compounds having one or more ethylenically unsaturated groups have a pronounced tendency to undergo free radical polymerization. Such compounds are therefore also referred to below as polymerizable compounds. The tendency of these compounds to undergo free radical polymerization means that they are used as monomers for producing polymers. However, the pronounced tendency of these compounds to undergo free radical polymerization is also a drawback. This is because undesirable spontaneous free radical polymerization can occur during storage and transportation as well as during chemical and physical processing, for example distillation or rectification, especially under the action of energy such as heat and / or light. As a result of such uncontrolled polymerization, for example on heated surfaces, polymer deposits can gradually form, which makes it necessary to remove the polymer deposits and therefore often leads to reduced operating times. Uncontrolled polymerization can even proceed explosively.

[0003] Therefore, during storage and transportation, as well as during chemical and physical processing, of ethylenically unsaturated compounds that have a tendency to undergo free radical polymerization, or mixtures that contain such compounds, it is customary to add compounds that prevent or at least retard the undesired spontaneous free radical polymerization. Such substances are often called polymerization inhibitors.

[0004] Polymerization inhibitors can be used as individual compounds or as mixtures of compounds. Polymerization inhibitors are subject to certain requirements depending on their field of use. For a polymerization inhibitor to be suitable as a transport and / or storage stabilizer for ethylenically unsaturated compounds, it is important that the efficiency of the polymerization inhibitor, i.e. the degree of inhibition of polymerization, is controllable. Under the conditions of storage and / or transportation of ethylenically unsaturated compounds, the polymerization inhibitor should sufficiently prevent or retard undesired spontaneous free radical polymerization, but under suitable polymerization conditions, the desired free radical polymerization of ethylenically unsaturated compounds should be possible without the need to first separate the polymerization inhibitor used during storage and / or transportation. If the stabilizer used during storage and / or transportation is not separated for the desired polymerization, it is important that this stabilizer does not adversely affect the desired polymerization, e.g. does not act unintentionally as a chain transfer agent.

[0005] In terms of world production, acrylic acid is undoubtedly one of the most important ethylenically unsaturated compounds. Acrylic acid is usually stabilized against undesired spontaneous free radical polymerization during storage and / or transportation with 0.018% to 0.022% by weight of hydroquinone monomethyl ether (MEHQ) relative to the amount of acrylic acid. In order to adequately stabilize acrylic acid against undesired spontaneous free radical polymerization using MEHQ, it is necessary that a sufficient amount of oxygen is dissolved in the acrylic acid. If acrylic acid is stored and / or transported in an atmosphere containing 5% to 21% by volume of oxygen, a sufficient amount of oxygen generally dissolves in the acrylic acid. In the desired polymerization of acrylic acid, the content of dissolved oxygen in the acrylic acid decreases, thereby decreasing the efficiency of MEHQ as a polymerization inhibitor sufficient for acrylic acid to polymerize in the presence of MEHQ.

[0006] In addition to its use as a polymerization inhibitor for the storage and / or transport of acrylic acid, MEHQ is also used as a polymerization inhibitor for the storage and / or transport of methacrylic acid, acrylic acid esters, and / or methacrylic acid esters, or mixtures containing one or more of said compounds.

[0007] Due to its widespread use, MEHQ has become one of the most important storage and / or transport stabilizers for ethylenically unsaturated compounds, especially acrylic acid, methacrylic acid, acrylic acid esters, and methacrylic acid esters.

[0008] The ethylenically unsaturated compounds are stored in suitable permanently installed containers, such as storage tanks (see, for example, Acrylic Acid, A Summary of Safety and Handling, 4th edition 2013, 7 Bulk Storage Facilities and Accessories). Preferably, the storage of the ethylenically unsaturated compounds in each container lasts for at least 1 hour, more preferably at least 10 hours, and most preferably at least 100 hours. Although the storage period under suitable conditions is theoretically unlimited, for economic reasons, the storage period is generally reduced to a minimum. Preferably, the storage of the ethylenically unsaturated compounds in each container lasts for no more than 180 days, more preferably no more than 90 days, and most preferably no more than 30 days. Particularly preferred ranges result from any combination of the above lower and upper limits.

[0009] The ethylenically unsaturated compounds are typically transported in suitable transportable containers, such as tanks or drums, by ship, rail, and / or truck (see, for example, Acrylic Acid, A Summary of Safety and Handling, 4th edition 2013, 9 Safe Transport of Acrylic Acid). The transportable metals may of course be permanently installed in the corresponding transport means, such as a tanker ship or a railroad tank car. Of course, it is also possible to store the containers at a specific location for a certain time before transporting them to another location. The ethylenically unsaturated compounds may also be transported, for example, by pipeline or hose to a storage tank, after purification of the ethylenically unsaturated compounds, when they are transferred from a storage tank to a transportable container and / or when they are transferred from one transportable container to another transportable container. Preferably, the transport of the ethylenically unsaturated compounds in each container lasts for at least 1 hour, more preferably at least 10 hours, and most preferably at least 100 hours. Although the period under suitable conditions is theoretically unlimited, for economic and safety reasons, the period is generally reduced to a minimum. The shipment of the ethylenically unsaturated compound in each container is preferably for a period of not more than 180 days, more preferably not more than 90 days, and most preferably not more than 30 days. Particularly preferred ranges result from any combination of the above lower and upper limits.

[0010] EP 0 850 916 A1 discloses a mixture of polymerization inhibitors for basic monomers, which mixture comprises a phenol. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] European Patent Application Publication No. 0850916 [Non-patent literature]

[0012] [Non-Patent Document 1] Acrylic Acid,A Summary of Safety and Handling,4th edition 2013,7 Bulk Storage Facilities and Accessories [Non-Patent Document 2] Acrylic Acid,A Summary of Safety and Handling,4th edition 2013,9 Safe Transport of Acrylic Acid Summary of the Invention [Problem to be solved by the invention]

[0013] The object of the present invention was to provide a method for storing and / or transporting ethylenically unsaturated compounds. The polymerization inhibitor ensures sufficient stabilization of the ethylenically unsaturated compounds against undesired free radical polymerization. However, in the intended free radical polymerization, it is not necessary to separate the polymerization inhibitor from the mixture before polymerization. Thus, the polymerization inhibitor does not act as a chain transfer agent and / or polymerization inhibitor in the intended free radical polymerization. Furthermore, the polymerization inhibitor does not result in undesired discoloration during the course of polymerization. [Means for solving the problem]

[0014] The object is to provide a method for the storage and / or transport of ethylenically unsaturated compounds, the method comprising the steps of: (a) storing and / or transporting ethylenically unsaturated compounds having a purity of at least 90% by weight; (b) storing and / or transporting the ethylenically unsaturated compounds in a mass of at least 100 kg; (c) storing and / or transporting the ethylenically unsaturated compounds in a mass of at least 100 kg; (d) storing and / or transporting the ethylenically unsaturated compounds in a mass of at least 100 kg; (e) storing and / or transporting the ethylenically unsaturated compounds in a mass of at least 100 kg; (f) storing and / or transporting the ethylenically unsaturated compounds in a mass of at least 100 kg; (g) storing and / or transporting the ethylenically unsaturated compounds in a mass of at least 100 kg; [ka] The compound includes During the ceremony, R 1 , R 3 , and R 5 is independently methyl or ethyl, preferably methyl; R 2 and R 4 are independently H, methyl, or ethyl, preferably H or methyl, more preferably H; This is achieved by the method. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Suitable compounds of general formula (I) are, for example, 2,4,6-trimethylphenol (TMP), 2-ethyl-4,6-dimethylphenol, 2,3,4,6-tetramethylphenol, 3-ethyl-2,4,6-trimethylphenol, 2-ethyl-3,4,6-trimethylphenol, 2,3-diethyl-4,6-dimethylphenol, 4-ethyl-2,6-dimethylphenol, 2,4-diethyl-6-methylphenol, 4-ethyl-2,3,6-trimethylphenol, 3,4-diethyl-2,6-dimethylphenol, 2,4-diethyl-3,6-dimethylphenol, 2,3,4-triethyl-6-methylphenol, 2,4,5,6-tetramethylphenol, 2-ethyl-4,5,6-trimethylphenol, 2,3,4,5,6-pentamethylphenol, 3-ethyl-2,4,5,6-tetramethylphenol, 2-ethyl-3,4,5,6-tetramethylphenol, 2,3-diethyl-4,5,6-trimethylphenol, 4-ethyl-2,5,6-trimethylphenol, 2,4-diethyl-5,6-dimethylphenol, 4-ethyl-2,3,5,6-tetramethylphenol, 3,4-diethyl-2,5,6-trimethylphenol, 2,4-diethyl-3,5,6-trimethylphenol, 2,3,4-triethyl-5,6-dimethylphenol, 5-ethyl-2,4,6-trimethylphenol, 2,5-diethyl-4,6-dimethylphenol, 5-ethyl-2,3,4,6-tetramethylphenol, 3,5-diethyl-2,4,6-trimethylphenol, 2,5-diethyl-3,4,6-trimethylphenol, 2,3,5-triethyl-4,6-dimethylphenol, 4,5-diethyl-2,6-dimethylphenol, 2,4,5-triethyl-6-methylphenol, 4,5-diethyl-2,3,6-trimethylphenol, 3,4,5-triethyl-2,6-dimethylphenol, 2,4,5-triethyl-3,6-dimethylphenol, 2,3,4,5-tetraethyl-6-methylphenol, 6-ethyl-2,4-dimethylphenol, 2,6-diethyl-6-methylphenol, 6-ethyl-2,3,4-trimethylphenol, 3,6-diethyl-2,4-dimethylphenol, 2,6-diethyl-3,4-dimethylphenol, 2,3,6-triethyl-4-methylphenol, 4,6-diethyl-2-methylphenol, 2,4,6-triethylphenol, 4,6-diethyl-2,3-dimethylphenol, 3,4,6-triethyl-2-methylphenol, 2,4,6-triethyl-3-methylphenol, 2,3,4,6-tetraethylphenol, 6-ethyl-2,4,5-trimethylphenol, 2,6-diethyl-4,5-dimethylphenol, 6-ethyl-2,3,4,5-tetramethylphenol, 3,6-diethyl-2,4,5-trimethylphenol, 2,6-diethyl-3,4,5-trimethylphenol, 2,3,6-triethyl-4,5-dimethylphenol, 4,6-diethyl-2,5-dimethylphenol, 2,4,6-triethyl-5-methylphenol, 4,6-diethyl-2,3,5-trimethylphenol, 3,4,6-triethyl-2,5-dimethylphenol, 2,4,6-triethyl-3,5-dimethylphenol, 2,3,4,6-tetraethyl-5-methylphenol, 5,6-diethyl-2,4-dimethylphenol, 2,5,6-triethyl-4-methylphenol, 5,6-diethyl-2,3,4-trimethylphenol, 3,5,6-triethyl-2,4-dimethylphenol, 2,5,6-triethyl-3,4-dimethylphenol, 2,3,5,6-tetraethyl-4-methylphenol, These are 4,5,6-triethyl-2-methylphenol, 2,4,5,6-tetraethylphenol, 4,5,6-triethyl-2,3-dimethylphenol, 3,4,5,6-tetraethyl-2-methylphenol, 2,4,5,6-tetraethyl-3-methylphenol, and 2,3,4,5,6-pentaethylphenol.

[0016] The purity of the ethylenically unsaturated compounds is preferably at least 95% by weight, more preferably at least 98% by weight, and most preferably at least 99% by weight.

[0017] The mass of the ethylenically unsaturated compound to be stored and / or transported is preferably at least 200 kg, more preferably at least 500 kg, and most preferably at least 1000 kg. The mass of the ethylenically unsaturated compound to be stored and / or transported is typically less than 10 million kg.

[0018] The ethylenically unsaturated compound during storage and / or transportation preferably has a temperature below 45° C., more preferably below 40° C., and most preferably below 35° C. In the case of acrylic acid, the temperature during storage and / or transportation should be at least 15° C.

[0019] The ethylenically unsaturated compounds preferably comprise compounds containing amide groups, esters of phosphorous acid, phosphoric acids, or phosphines, in each case less than 0.0001% by weight of each, relative to the total amount of ethylenically unsaturated compounds.

[0020] In a preferred embodiment of the invention, the compound of general formula (I) is used essentially as the only polymerization inhibitor.

[0021] The ethylenically unsaturated compound contains preferably 0.0002% by weight to 0.0600% by weight (2 to 600 ppm), more preferably 0.0005% by weight to 0.0450% by weight (5 to 450 ppm), and most preferably 0.0010% by weight to 0.0300% by weight (10 to 300 ppm) of the compound of general formula (I) relative to the total amount of the ethylenically unsaturated compounds in any case.

[0022] The ethylenically unsaturated compound is preferably a mono-, di-, or triethylenically unsaturated C3-C8 carboxylic acid, a mono-, di-, or triethylenically unsaturated C3-C8 carboxylic acid ester having 1 to 20 carbon atoms in the ester group.

[0023] Mono-, di-, or triethylenically unsaturated C3-C6 carboxylic acids, such as acrylic acid, methacrylic acid, dimethacrylic acid, ethacrylic acid, citraconic acid, methylenemalonic acid, crotonic acid, fumaric acid, mesaconic acid, itaconic acid, and maleic acid; mono-, di-, or triethylenically unsaturated C3-C6 carboxylic acid esters having 1 to 12 carbon atoms in the ester group, such as C1-C 12 Alkyl-containing acrylic esters, C1-C 12 Methacrylic acid esters with alkyl groups, C1-C 12 Dimethacrylate esters with alkyl groups, C1-C 12 Ethacrylic acid esters with alkyl groups, C1-C 12 Citraconic acid esters with alkyl groups, C1-C 12 Methylenemalonate esters with alkyl groups, C1-C 12 Alkyl crotonates, C1-C 12 Alkyl fumarate esters, C1-C 12 Mesaconic acid esters with alkyl groups, C1-C 12 Itaconic acid esters with alkyl groups, and C1-C 12 Alkyl-containing maleates are particularly preferred.

[0024] Particularly preferred are acrylic acid, methacrylic acid, acrylic acid esters having C1-C8 alkyl groups such as methyl acrylate, ethyl acrylate, n-butyl acrylate, and 2-ethylhexyl acrylate, and methacrylic acid esters having C1-C8 alkyl groups such as methyl methacrylate.

[0025] Further suitable ethylenically unsaturated compounds are dipropylene glycol diacrylate, tripropylene glycol diacrylate, polyethylene glycol diacrylate, glycerol triacrylate, ethoxylated glycerol triacrylate, trimethylolpropane triacrylate, ethoxylated trimethylolpropane triacrylate, butanediol monoacrylate, dicyclopentadienyl acrylate, 2-dimethylaminoethyl acrylate, 2-hydroxyethyl acrylate, and 2-hydroxypropyl acrylate.

[0026] When acrylic acid and / or methacrylic acid are used as the polymerizable compound, the total amount of the compound of general formula (I) in the mixture is preferably 0.0050% by weight to 0.0600% by weight (50 to 600 ppm), more preferably 0.0100% by weight to 0.0450% by weight (100 to 450 ppm), and even more preferably 0.0150% by weight to 0.0300% by weight (150 to 300 ppm), relative to the total amount of acrylic acid and / or methacrylic acid in any case.

[0027] Acrylic acid and / or methacrylic acid are typically stored and / or transported in stainless steel containers. A suitable stainless steel container contains 16.5% to 19.5% by weight chromium and 8.0% to 13.5% by weight nickel.

[0028] When methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate, and / or 2-ethylhexyl acrylate is used as the polymerizable compound, the total amount of the compound of general formula (I) in the mixture is preferably 0.0002% by weight to 0.0060% by weight (2 to 60 ppm), more preferably 0.0005% by weight to 0.0040% by weight (5 to 40 ppm), and even more preferably 0.0010% by weight to 0.0020% by weight (10 to 20 ppm), relative to the total amount of methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate, and / or 2-ethylhexyl acrylate.

[0029] Ethylenically unsaturated compounds are typically stored and / or transported in an oxygen-containing atmosphere.

[0030] The ethylenically unsaturated compound is preferably stored in a container in an oxygen-containing atmosphere having an oxygen content of 5% to 10% by volume, and the mixture in the container is periodically recirculated at least once a week, for example by pumping recirculation of the entire tank contents. This relatively low oxygen content prevents the formation of an ignitable gas mixture in the container. Recirculation replenishes the dissolved oxygen consumed in the liquid ethylenically unsaturated compound.

[0031] The present invention further provides a method of free radical polymerization in which at least one of the above ethylenically unsaturated compounds is polymerized by a polymerization initiator, and the use of one of the above ethylenically unsaturated compounds for free radical polymerization by a polymerization initiator.

[0032] The free radical polymerization of the present invention can provide, for example, water-soluble and water-swellable polyacrylic acid and its sodium salt.

[0033] Free radical polymerization itself is common knowledge and is typically carried out in solution. Suitable polymerization initiators are all compounds that can decompose into free radicals under the selected reaction conditions, such as thermal initiators, redox initiators, photoinitiators. Suitable thermal initiators are peroxo mono- and disulfates, as well as peroxo mono- and diphosphates. Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium hypophosphite, sodium peroxodisulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. EXAMPLES

[0034] Polymerization inhibition experiment The ethylenically unsaturated compounds used were distilled twice to remove the hydroquinone monomethyl ether (MEHQ) polymerization inhibitor. Defined amounts of the indicated polymerization inhibitors were added to the ethylenically unsaturated compounds in each case. MEHQ and 2,4,6-trimethylphenol (TMP) were used.

[0035] 0.5 ml of each mixture was transferred into 1.8 ml ampoules and stored in an air circulating oven at the specified temperature.

[0036] In each test series, at least three ampoules of each mixture were tested and the average time required for complete polymerization was determined by visual inspection.

[0037] [Table 1]

[0038] [Table 2]

[0039] [Table 3]

[0040] [Table 4]

[0041] 2,4,6-trimethylphenol (TMP) has an inhibitory effect comparable to a clearly better inhibitory effect on MEHQ.

[0042] Polymerization experiments Polymerization of acrylic acid The reaction vessel was initially charged with 450 g of water under a nitrogen atmosphere. This initial charge reaction mixture was heated with stirring to 95° C. Once a temperature of 95° C. was reached, the three streams were metered in with stirring while maintaining the temperature. Flow 1: 500 g of acrylic acid metered in over 5 hours Stream 2: 15 g sodium hypophosphite in 35 g deionized water, metered in over 4.75 hours Stream 3: 5 g sodium peroxodisulfate in 66.4 g deionized water, metered in over 5.25 hours

[0043] After the three streams were added, the reaction mixture was stirred for an additional hour at 95° C. The reaction mixture was then cooled to room temperature and 80 g of water was added.

[0044] The acrylic acid used in stream 1 was stabilized with MEHQ or 2,4,6-trimethylphenol (TMP).

[0045] The obtained polymer was analyzed by GPC (calibration with Na-PAA standards, eluent 0.01 mol / l phosphate buffer pH 7.4, 0.01 M NaN3 in distilled water).

[0046] [Table 5]

[0047] The concentrations of 2,4,6-trimethylphenol (TMP) tested result in polyacrylic acid with molar masses comparable to those obtained using the standard concentration of MEHQ.

[0048] Polymerization of sodium acrylate or sodium methacrylate Under a nitrogen atmosphere, a reaction vessel was charged with 52 g of deionized water and 60 g of an aqueous solution of sodium acrylate or sodium methacrylate (in each case with a concentration of 37.2 wt % stabilized with 0.01 wt % MEHQ or TMP). Then, 0.4 g of sodium peroxodisulfate dissolved in 10 g of deionized water was added and the mixture was heated to 80° C. with stirring. The reaction mixture was stirred at 80° C. for 4 hours and then cooled to ambient temperature.

[0049] In all experiments with MEHQ, the color of the solution changed during heating (at about 60°C).

[0050] [Table 6]

[0051] The colour number was determined according to DIN EN ISO 6271 (2005).

[0052] 2,4,6-trimethylphenol (TMP) causes undesirable discoloration to a much lesser extent than when MEHQ is used.

Claims

1. 1. A method for storing and / or transporting an ethylenically unsaturated compound, comprising: the ethylenically unsaturated compound has a purity of at least 90% by weight; the mass of the ethylenically unsaturated compound to be stored and / or transported is at least 100 kg; The temperature during storage and / or transportation of the ethylenically unsaturated compound is less than 50° C.; the ethylenically unsaturated compound is a carboxylic acid or a carboxylic acid ester, The ethylenically unsaturated compound is 0.0001% by weight to 0.0750% by weight of a compound represented by the general formula (I): 【Chemistry 1】 The compound includes During the ceremony, R １ , R ３ , and R ５ is independently methyl or ethyl; R ２ and R ４ is independently H, methyl, or ethyl.

2. 2. The method of claim 1, wherein the ethylenically unsaturated compounds comprise less than 0.0001% by weight of a compound containing an amide group, an ester of phosphorous acid, phosphoric acid, or a phosphine, in each case relative to the total amount of ethylenically unsaturated compounds.

3. 2. The method according to claim 1, wherein the compound of general formula (I) is used essentially as the only polymerization inhibitor.

4. The method according to any one of claims 1 to 3, wherein 2,4,6-trimethylphenol is used as the compound of general formula (I).

5. 5. The method of any one of claims 1 to 4, wherein the storage and / or transportation of the ethylenically unsaturated compound is for a period of at least 10 hours.

6. 6. The method according to claim 1, wherein the ethylenically unsaturated compound is acrylic acid and / or methacrylic acid.

7. 7. The method according to claim 6, wherein the ethylenically unsaturated compounds comprise 0.0050% to 0.0600% by weight of the compound of general formula (I) relative to the total amount of ethylenically unsaturated compounds.

8. 7. The method according to claim 6, wherein the ethylenically unsaturated compounds comprise 0.0150% to 0.0300% by weight of the compound of general formula (I) relative to the total amount of ethylenically unsaturated compounds.

9. 6. The method of any one of claims 1 to 5, wherein the ethylenically unsaturated compound is methyl acrylate, methyl methacrylate, ethyl acrylate, n-butyl acrylate, and / or 2-ethylhexyl acrylate.

10. 10. The method according to claim 9, wherein the ethylenically unsaturated compounds comprise 0.0002% to 0.0060% by weight of the compound of general formula (I) relative to the total amount of ethylenically unsaturated compounds.

11. 10. The method according to claim 9, wherein the ethylenically unsaturated compounds comprise 0.0010% to 0.0020% by weight of the compound of general formula (I) relative to the total amount of ethylenically unsaturated compounds.

12. 12. The method according to any one of claims 1 to 11, wherein the ethylenically unsaturated compound is stored and / or transported under an oxygen atmosphere.

13. 13. The method according to any one of claims 1 to 12, wherein the ethylenically unsaturated compound is stored in a container under an oxygen-containing atmosphere having an oxygen content of 5% to 10% by volume, and the ethylenically unsaturated compound in the container is periodically recirculated.

14. 1. A method of free radical polymerization comprising the steps of: At least one ethylenically unsaturated compound is polymerized by a polymerization initiator; the ethylenically unsaturated compound is a carboxylic acid or a carboxylic acid ester, the ethylenically unsaturated compound used has a purity of at least 90% by weight, The ethylenically unsaturated compound is 0.0001% by weight to 0.0750% by weight of a compound represented by the general formula (I): 【Chemistry 2】 The compound includes During the ceremony, R １ , R ３ , and R ５ is independently methyl or ethyl; R ２ and R ４ is independently H, methyl, or ethyl.

15. Use of an ethylenically unsaturated carbonyl compound for free radical polymerization with a polymerization initiator, comprising: the ethylenically unsaturated compound is a carboxylic acid or a carboxylic acid ester, the ethylenically unsaturated compound has a purity of at least 90% by weight; The ethylenically unsaturated compound is 0.0001% by weight to 0.0750% by weight of a compound represented by the general formula (I): 【Chemistry 3】 The compound includes During the ceremony, R １ , R ３ , and R ５ is independently methyl or ethyl; R ２ and R ４ is independently H, methyl, or ethyl.

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